Stage 6 · Big numbers & speed
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
Two numbers with the same tens digit whose ones make 10: 23 × 27 → front: the tens digit 2 times the number after it, 3, 2 × 3 = 6; back: the two ones digits multiplied, 3 × 7 = 21; front then back → 621. The back is always two digits: 21 × 29 → 6 and 09 → 609. Squares ending in 5 (35² → 1225) are this same trick. You can also multiply in columns (23 × 7 = 161, 23 × 20 = 460, 161 + 460 = 621); the answer is the same.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
27 × 23
Show the answer and working
Answer: 27 × 23 = 621
27 × 23: the same tens, and the ones make 7 + 3 = 10. Front 2 × 3 = 6, back 7 × 3 = 21, front then back → 621
Example 2
14 × 16
Show the answer and working
Answer: 14 × 16 = 224
14 × 16: the same tens, and the ones make 4 + 6 = 10. Front 1 × 2 = 2, back 4 × 6 = 24, front then back → 224
Example 3
93 × 97
Show the answer and working
Answer: 93 × 97 = 9021
93 × 97: the same tens, and the ones make 3 + 7 = 10. Front 9 × 10 = 90, back 3 × 7 = 21, front then back → 9021
Example 4
31 × 39
Show the answer and working
Answer: 31 × 39 = 1209
31 × 39: the same tens, and the ones make 1 + 9 = 10. Front 3 × 4 = 12, back 1 × 9 = 9, the back is always two digits: 9 is written 09, front then back → 1209
Practise same tens, ones make 10 in the game →Stage 6 · Big numbers & speed · Free to start, unlimited questions, no download