Round, then adjust: a mental math trick
When to use it: A number sits just below a round one, like 29, 98 or $1.99.
Worked example
- Use the round number instead.
- Work out the answer.
- Put back what the rounding changed. In a product, multiply the difference by the other number: 29 × 4 = 120 − 4.
83 − 29
- 83 − 30 = 53.
- You took away 1 too many, so add it back: 53 + 1 = 54.
Answer: 54
Try three
-
123 + 28
Show the answer
151
-
59 × 4
Show the answer
236
-
29 × 4
Show the answer
116