Multiplying and dividing by two digits; divisibility · Usually met around P4
Multiplying by 10, by 100 and by whole tens
Multiply the digit, then add the 0.
How to do it
Method
Multiply the digit, then add the 0
34 × 20 = 34 × 2 × 10 = 680
Another way
Times 10 first
34 × 10 × 2 = 340 × 2 = 680
In full
Times 10, every digit moves one place to the left and a 0 goes on the end: 34 × 10 = 340. Times 100, two 0s: 34 × 100 = 3400. To multiply by whole tens, multiply by the tens digit, then by 10: 34 × 20 → 34 × 2 = 68, 68 × 10 = 680. Whole hundreds are the same: 34 × 300 = 34 × 3 × 100 = 10200. Careful with 25 × 40: 25 × 4 = 100 already ends in two 0s, and one more makes 1000. Another way: times 10 first, then by the tens digit: 34 × 20 = 34 × 10 × 2 = 340 × 2 = 680, the same. To multiply by a two-digit number, split it into whole tens and ones, multiply each and add: 34 × 21 = 34 × 20 + 34 × 1 = 680 + 34 = 714. That is how long multiplication works.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
68 × 600 = □
Show the answer and working
Answer: 40800
68 × 6 = 408, then two 0s on the end: 40800.
Example 2
94 × 60 = 5640
True or false?
Show the answer and working
Answer: True
94 × 6, then times 10: 94 × 60 = 5640.
Example 3
51 × □ = 2550, where □ is a whole number of tens.
Show the answer and working
Answer: 50
2550 ÷ 51 = 50. Check: 51 × 50 = 2550.
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