Multiplying and dividing by two digits; divisibility · Usually met around P4
Same tens, ones make 10
Tens × the next number, then ones × ones.
How to do it
Method
Tens × the next number, then ones × ones
23 × 27: 2 × 3 = 6, 3 × 7 = 21 → 621
Another way
Column multiplication
23 × 7 = 161, 23 × 20 = 460, 161 + 460 = 621
In full
This trick is for two numbers with the same tens digit whose ones digits make 10. 23 × 27 → front: the tens digit 2 times the number after it, 3, so 2 × 3 = 6. Back: multiply the two ones digits, 3 × 7 = 21. Put the front and back together → 621. The back always has two digits: 21 × 29 → 6 and 09 → 609. Squares ending in 5 (35² → 1225) use this same trick. You can also multiply in columns (23 × 7 = 161, 23 × 20 = 460, 161 + 460 = 621).
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
12 × 18
Show the answer and working
Answer: 12 × 18 = 216
12 × 18: the same tens, and the ones make 2 + 8 = 10. Front 1 × 2 = 2, back 2 × 8 = 16, front then back → 216
Example 2
23 × 27
Show the answer and working
Answer: 23 × 27 = 621
23 × 27: the same tens, and the ones make 3 + 7 = 10. Front 2 × 3 = 6, back 3 × 7 = 21, front then back → 621
Example 3
22 × 28
Show the answer and working
Answer: 22 × 28 = 616
22 × 28: the same tens, and the ones make 2 + 8 = 10. Front 2 × 3 = 6, back 2 × 8 = 16, front then back → 616
Practise it in the game →Multiplying and dividing by two digits; divisibility · Free to start, unlimited questions, no download