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Multiplying, dividing and mixed operations · Usually met around P4

Multiplying and dividing by two digits; divisibility

From “Multiplying by 10, by 100 and by whole tens” to “Divisibility rules for 4, 6, 8, 9, 25 and 100”, step by step, each skill with its methods, worked examples and answers.

Skills in this topic, easy to hard

  1. Multiplying by 10, by 100 and by whole tensMultiply the digit, then add the 0
  2. Long multiplicationSplit the multiplier into tens and ones
  3. Reordering and regrouping factorsPair up first: 25 × 4, 125 × 8
  4. ×5 by halving×5 is ×10, then halve
  5. Two-digit × 11Split the digits, put their sum between
  6. Squares to 25Ends in 5: tens × next number, then 25
  7. Square and cube numbers²: two of it multiplied; ³: three
  8. Same tens, ones make 10Tens × the next number, then ones × ones
  9. Multiplying near 100Front: cross-subtract. Back: gap × gap
  10. Long division (by two digits)Round the divisor to a ten, then adjust
  11. Four-digit numbers times and divided by two digitsTimes: split into tens and ones. Divide: try a digit; a remainder can be a fraction
  12. Estimate first, then checkRound and estimate, then judge the answer
  13. Divisibility rules for 2, 3, 5 and 102, 5, 10: the last digit. 3: the digit sum
  14. Divisibility rules for 4, 6, 8, 9, 25 and 1004, 25, 100: last two digits. 8: last three. 9: the digit sum. 6: both 2 and 3
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