Stage 6 · Big numbers & speed
×25 and ×125
×25 is ×100, then halve twice.
How to do it
Method
×25 is ×100, then halve twice
37 × 25 = 3700 ÷ 2 ÷ 2 = 1850 ÷ 2 = 925
Another way
Column multiplication
37 × 5 = 185, 37 × 20 = 740, 185 + 740 = 925
In full
×25 is ×100, then ÷ 4: 37 × 25 → 3700, halved twice: 1850, 925. If the number divides by 4, ÷ 4 first is quicker: 36 × 25 → 36 ÷ 4 = 9 → 900. ×125 is ×1000, then ÷ 8 — halve three times: 13 × 125 → 13000 → 6500 → 3250 → 1625; if it divides by 8, ÷ 8 first: 24 × 125 → 24 ÷ 8 = 3 → 3000. You can also multiply in columns, or reorder using 25 × 4 = 100 and 125 × 8 = 1000; the answer is the same.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
97 × 25
Show the answer and working
Answer: 97 × 25 = 2425
97 × 25: ×25 is ×100, then ÷ 4 — halve twice, 97 × 100 = 9700, 9700 ÷ 2 = 4850, 4850 ÷ 2 = 2425
Example 2
91 × 25
Show the answer and working
Answer: 91 × 25 = 2275
91 × 25: ×25 is ×100, then ÷ 4 — halve twice, 91 × 100 = 9100, 9100 ÷ 2 = 4550, 4550 ÷ 2 = 2275
Example 3
6 × 125
Show the answer and working
Answer: 6 × 125 = 750
6 × 125: ×125 is ×1000, then ÷ 8 — halve three times, 6 × 1000 = 6000, 6000 ÷ 2 = 3000, 3000 ÷ 2 = 1500, 1500 ÷ 2 = 750
Example 4
23 × 125
Show the answer and working
Answer: 23 × 125 = 2875
23 × 125: ×125 is ×1000, then ÷ 8 — halve three times, 23 × 1000 = 23000, 23000 ÷ 2 = 11500, 11500 ÷ 2 = 5750, 5750 ÷ 2 = 2875
Practise ×25 and ×125 in the game →Stage 6 · Big numbers & speed · Free to start, unlimited questions, no download