Patterns & Reasoning

Problem

A denominator that cancels to 1 leaves a whole number

An expression multiplies and divides a mixed number, leaving one blank. We need the natural number in the blank that makes the whole expression the smallest possible whole number.
Number system
Your answer
How to solve
Strategy Identify Subproblems — Tidy the expression down to a single fraction first. Once everything that can cancel has cancelled, what is left is one denominator, and the question becomes the much easier one of what makes that denominator disappear.
1STEP 1

Turn the mixed number into a fraction

A mixed number cannot be cancelled while it is still split into a whole part and a fraction, so write it as a single improper fraction first.

2 3/4 = 11/4
2STEP 2

Cancel the 11 and see what is left

Dividing by 11 is multiplying by 1/11, and the 11 on top cancels it. Every part of the expression is now gone except a single quarter of the blank.

11/4 ÷ 11 × ■ = ■/4
3STEP 3

Decide what makes a quarter whole

A fraction with denominator 4 is a whole number exactly when the numerator is a multiple of 4, so the blank has to be 4, 8, 12, and so on.

■ = 4, 8, 12, …
4STEP 4

Take the smallest one

The results for those blanks are 1, 2, 3, and so on, so the smallest whole-number result comes from the smallest multiple of 4.

■ = 4 → 4/4 = 1
Answer
4
Check it: 2 3/4 ÷ 11 × 4 = 11/4 × 1/11 × 4 = 1, a whole number.
Takeaway

Cancel everything you can first -- what is left over is usually a much smaller question.

  • Turn the mixed number into a fraction
  • Cancel the 11 and see what is left
  • Decide what makes a quarter whole
  • Take the smallest one

▶ Practice — 12 problems