Picking the right math tool

Mathematics · Mathematical Thinking · ages 8-9

Name ______________________ Date ____________

Work out 25 x 4 mentally.
Answer: ______________
Which job suits mental maths best?
- 167 x 3
- 25 x 4
- 458 x 7
- 1296 x 8
Which is easier to solve in your head: 20 × 5 or 238 × 7?
- 20 × 5
- 238 × 7
- They are equally easy
Drawing a rectangle on squared paper and counting the squares is a good way to check an area you calculated.
Circle one: True False
A rectangle is 5 cm long and 3 cm wide. Counting unit squares gives its area. What is it in square centimetres?
Answer: ______________
Maria multiplies 4 × 3 to find the area of a rectangle. She draws it on squared paper, shown here. Count the squares to check her answer.
Answer: ______________
To compare 3/8 and 1/4, 1/4 is changed to 2/8. Which fraction is bigger?
- 1/4
- They are equal
- 3/8
- Cannot tell from strips
Ava calculates length x width as 15, then counts 15 squares on squared paper. She says counting checks the calculation. Is Ava right?
Circle one: True False
Maya writes out every multiplication on paper, even 10 × 2. What would save her time?
- Using a fraction strip
- Using mental math for friendly numbers like 10 × 2
- Measuring with a ruler
Work out 167 x 3 with a written method.
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

100 · 25 x 4 = 100 because four quarters make a whole hundred.
25 x 4 · 25 x 4 suits mental maths because 25 splits cleanly into quarters of 100. The others tangle and deserve written methods.
20 × 5 · 20 × 5 has friendly, round numbers. 238 × 7 has bigger digits that are easy to slip up on without writing them down.
True · Squared paper lets you count and confirm that length times width gives the right number of squares.
15 · The area is 15 square centimetres because 5 x 3 = 15 and the count of squares agrees.
12 · 4 rows of 3 squares make 12 squares in all, which matches 4 × 3.
3/8 · 3/8 is bigger because 3 pieces of 8 beat 2 pieces of 8.
True · Ava is right because two tools agreeing means the answer is almost certainly correct.
Using mental math for friendly numbers like 10 × 2 · 10 × 2 is simple enough to solve in her head, so writing it out just takes extra time.
501 · 167 x 3 = 501 because 100 x 3 = 300, 60 x 3 = 180 and 7 x 3 = 21, and 300 + 180 + 21 = 501.