Multiply by 5
Multiply by 10, then halve. 48 × 5 → 480 ÷ 2 = 240
Multiply by 9
Multiply by 10, then take away the number once. 37 × 9 → 370 − 37 = 333
Multiply a two-digit number by 11
Add the two digits and put the sum in the middle. 23 × 11 → 2 (2+3) 3 = 253
If the middle sum is 10 or more, carry the 1. 57 × 11 → 5 (12) 7 → 627
Square a number ending in 5
Multiply the first digit by the next number up, then write 25 after it. 35² → 3 × 4 = 12, then 25 → 1225
Round and adjust
Round one number to a friendly ten, then fix the difference. 48 + 37 → 50 + 37 − 2 = 85
This is exactly the big friend idea from Lesson 2: +9 is the same as +10 −1.
Double and halve
Halve one number and double the other – the answer stays the same. 16 × 25 → 8 × 50 → 4 × 100 = 400
Try it yourself
Use the ×11 trick to find 23 × 11 and show it on the abacus.
Quick check
Type your answers, then check them. Get them all right to complete the lesson automatically.
Practice tips
- Pick one trick per week and use it every day until it is automatic.
- Always check a trick answer another way the first few times.