Mental math with decimals

What they’re learning

Your child is using mental strategies with decimals, including multiplying by 0.1 or 0.01 and estimating decimal sums and differences.

Explain it to my child

Multiplying by 0.1 finds one tenth of a number. One tenth of 36 is 3.6; one hundredth of 36 is 0.36.

Try it together

Use prices or measurements. Ask for one tenth or one hundredth of friendly quantities such as 50, 24, or 300, then compare the mental answer with a calculator.

Practice

1. 24 × 0.1 = ?
2. 300 × 0.01 = ?
3. Estimate 7.82 + 4.11.
4. Mentally solve 9.9 + 2.4.
5. Estimate 20.03 - 8.88.

Check understanding

• Can your child explain why multiplying by 0.1 makes a positive number smaller?
• Can they choose friendly nearby decimals for estimation?