In many ways, mental calculation is a process of simplification. For example,
the problem 432 × 3 sounds hard, but it’s the sum of three easy problems:
3 × 400 = 1200, 3 × 30 = 90, and 3 × 2 = 6; 1200 + 90 + 6 = 1296. Notice
that when adding the numbers, it’s easier to add from largest to smallest,
rather than smallest to largest.
Again, doing mental calculations from left to right is also generally easier
because that’s the way we read numbers. Consider 54 × 7. On paper, you
might start by multiplying 7 × 4 to get 28, but when doing the problem
mentally, it’s better to start with 7 × 50 (350) to get an estimate of the answer.
To get the exact answer, add the product of 7 × 50 and the product of 7 × 4:
350 + 28 = 378.
the problem 432 × 3 sounds hard, but it’s the sum of three easy problems:
3 × 400 = 1200, 3 × 30 = 90, and 3 × 2 = 6; 1200 + 90 + 6 = 1296. Notice
that when adding the numbers, it’s easier to add from largest to smallest,
rather than smallest to largest.
Again, doing mental calculations from left to right is also generally easier
because that’s the way we read numbers. Consider 54 × 7. On paper, you
might start by multiplying 7 × 4 to get 28, but when doing the problem
mentally, it’s better to start with 7 × 50 (350) to get an estimate of the answer.
To get the exact answer, add the product of 7 × 50 and the product of 7 × 4:
350 + 28 = 378.
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