Question

Is 2.1 x 4.9 x 7.2 = 105.84 a reasonable estimate

Answer 1

No.

You'd estimate this as 2 × 5 × 7.2 = 72.

You only keep the decimal place in 7.2 because the othe factor is 10 and it helps reduce the error.

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The actual value is 2.1 × 4.9 × 7.2 = 74.088, nowhere close to 106.