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The question

21 × 10

21×10
21 × 10=10 × 21
10 × 5=50
10 × 16=160
50+160=210
21×10=210
  1. 21 × 10 means 21 groups with 10 in each one — 21 rows of 10. The answer counts everything in all of them, not one row.
  2. Added up, that is 10 written 21 times — that is a lot of adding. Turn it round: 10 × 21 is the same amount, and it is only 10 lots of 21 to add instead.
  3. If 10 × 21 will not come, split the 21 at five: 10 × 5 = 50, and 10 × 16 = 160. Together that is 50 + 160 = 210.
  4. So 21 × 10 = 210.

How it works.

  1. 01

    21 × 10 means 21 groups with 10 in each one — 21 rows of 10. The answer counts everything in all of them, not one row.

  2. 02

    Added up, that is 10 written 21 times — that is a lot of adding. Turn it round: 10 × 21 is the same amount, and it is only 10 lots of 21 to add instead.

  3. 03

    If 10 × 21 will not come, split the 21 at five: 10 × 5 = 50, and 10 × 16 = 160. Together that is 50 + 160 = 210.

  4. 04

    So 21 × 10 = 210.

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