# Sharing \$15.75 Worth of Puzzles

### Pattern Puzzle:

When I saw that 35 × 45 = 1575, I suspected a pattern. I made a chart to see if my suspicions were true, and they were! Can you look at the chart and tell me what that pattern is?

If you were able to see that pattern, then look at each of these. They have patterns because the numbers in 3 × 17, 4 × 16, 5 × 15, 6 × 14, and 7 × 13 have a relationship. What is that relationship?

### Factors of 1575:

We can use 35 × 45 = 1575 to make one of its many possible factor trees:

• 1575 is a composite number.
• Prime factorization: 1575 = 3 × 3 × 5 × 5 × 7, which can be written 1575 = 3² × 5² × 7.
• 1575 has at least one exponent greater than 1 in its prime factorization so √1575 can be simplified. Taking the factor pair from the factor pair table below with the largest square number factor, we get √1575 = (√225)(√7) = 15√7.
• The exponents in the prime factorization are 2, 2, and 1. Adding one to each exponent and multiplying we get (2 + 1)(2 + 1)(1 + 1) = 3 × 3 × 2 = 18. Therefore 1575 has exactly 18 factors.
• The factors of 1575 are outlined with their factor pair partners in the graphic below.

Can you use a different factor pair to create another factor tree for 1575? Will you always get 1575 = 3² × 5² × 7 in the end?

### Difference of Two Squares Puzzle:

1575 is the difference of two squares in NINE different ways:
788² – 787² = 1575,
264² – 261² = 1575,
160² – 155² = 1575,
116² – 109² = 1575,
92² – 83² = 1575,
60² – 45² = 1575,
48² – 27² = 1575,
44² – 19² = 1575, and
40² – 5² = 1575.

In money 1575¢ is represented as \$15.75. That’s the same as 63 quarters! Which of those differences of two squares is illustrated using quarters in the image below:

Which of the nine difference of two squares above is illustrated in the following image?

That image illustrates that \$15.75 is just one quarter away from the next perfect square dollar amount, \$16.00. Both 16 and 1600 are perfect squares. Can you make the rectangle below by moving just one row of quarters from the image above?

Moving that one row could help you notice that
8² – 1² = (8 – 1) × (8 + 1) = 63,
and might be the first step in understanding that  a² – b² = (a + b)(a – b) .

### Dividing Mixed Numbers Puzzles:

A quarter is 25¢. The reason a quarter is called a quarter is because it is a quarter or 1/4th of a dollar. We usually write dollar and cents together as decimals. A quarter is \$0.25.

Three quarters is 75¢ or \$0.75 and is 3/4ths of a dollar.

Two quarters is 50¢ or \$0.50 and is 2/4ths or one half of a dollar.

Representing 1575¢ in quarters can help you understand dividing mixed numbers like in the problem below:

The answer to both questions is the same! Now try this one:

You might not find this next example easy, but give it a look:

Why is 13 in the denominator of the answer to both questions when it didn’t appear in either question? Where did the 13 come from?

Now try writing and solving your own problem:

Working with money often seems like more fun than working with numbers. I hope you enjoyed these puzzles today.

# 825 Quarters Make Dividing by 25 Easy

Numbers ending in 00, 25, 50, and 75 can be divided evenly by 25. How much is 825 divided by 25? That quotient is the same as the answer to “how many quarters are in \$8.25?” (A quarter is ¼ of a dollar and is written .25 or 25¢.)

You probably could visualize the answer in your head even if I hadn’t included a picture! That’s why I often ask kids the how-many-quarters question when they are stumped dividing something by 25 . It seems that kids are always able to give the quotient after that question. Notice that “8.25 ÷ .25 =” and  “8 ¼ ÷ ¼ =” have the same answer, too. You can also ask that how-many-quarters question to find the answer when something is divided by .25 or ¼.

It would almost be as easy to divide \$8.26 or \$8.39 by 25. The quotient would be the same as the problem above but with some loose change becoming the remainder. Using money for division problems could even help kids better understand dividends, divisors, quotients, and remainders.

Here’s an example of a how-many-quarters type question that will help you divide by 75, .75 or ¾.

We can count and see that there are 11 sets of 3 quarters in \$8.25. That means that \$8.25 ÷ .75 is 11. It also means that 8¼ ÷ ¾ = 11.

Dividing by fractions can be a very abstract concept for students, and they may ask questions like, “What does 8¼ ÷ 1¼ even mean?” Again quarters come to the rescue! 5 quarters can be so much more friendly than 1¼ is. Shorthand for 5 quarters is the fraction, 5/4. Since they have the same denominators, dividing 8¼ by 1¼ is the same as dividing 33 by 5:

Kids think money is more fun than math, but money is just a subset of mathematics which is full of lots of other fun topics, too. Here are a couple of ways other educators have used money to teach a math topic.

Jen of Beyond Tradit’l Math shared her way to teach subtracting decimals using money. Her method will surely captivate any child who tries it and even make regrouping fun:

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Robert Kaplinsky uses several very short videos to keep students engaged without them actually touching any money. Check out the replies, too. Paula Beardell Krieg’s excellent \$1.00 art project is there, and that would be fun for anyone 2nd grade or older to do:

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Now back to the number 825.

Clearly 825 has to be divisible by both 5 AND 3 in order for (821, 823, 827, 829) to be the fourth prime decade, which it is.

• The last digit of 825 is 5, so it is divisible by 5.
• 8 + 2 + 5 = 15, a multiple of 3, so 825 is divisible by 3.
• 8 – 2 + 5 = 11, so 825 is divisible by 11.

All numbers ending in 00, 25, 50, or 75 can have their square roots simplified. If you were trying to simplify √825, you could visualize quarters in your mind to easily divide 825 by 25. Then √825 = (√25)(√33) = 5√33

Here is 825’s factoring information:

• 825 is a composite number.
• Prime factorization: 825 = 3 × 5 × 5 × 11, which can be written 825 = 3 × 5² × 11
• The exponents in the prime factorization are 1, 2, and 1. Adding one to each and multiplying we get (1 + 1)(2 + 1)(1 + 1) = 2 × 3 × 2 = 12. Therefore 825 has exactly 12 factors.
• Factors of 825: 1, 3, 5, 11, 15, 25, 33, 55, 75, 165, 275, 825
• Factor pairs: 825 = 1 × 825, 3 × 275, 5 × 165, 11 × 75, 15 × 55, or 25 × 33
• Taking the factor pair with the largest square number factor, we get √825 = (√25)(√33) = 5√33 ≈ 28.72281