Here is a number sense trick to help you calculate multiplication using mental math.
We want to find the product of 5 x 4 x 3 x 2 x 1. Using our number sense, we want to find numbers close to ten or multiples of ten to make the multiplication easier.
Here, we can find pairs of numbers that will multiply to be friendly numbers, such as 5 x 2 = 10 and 4 x 3 = 12.
Multiplying by ten is easy to compute using mental math, and anything times one is just itself.
We are left with 10 x 12 = 120, and so
5 x 4 x 3 x 2 x 1= 120.
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