Multiply by whole tens in columns — a shortcut for 2-digit multiplication when one factor is a multiple of ten

I show a shortcut for 2-digit multiplication problems when one of the factors is a multiple of ten, such as 78 × 50. In such cases the answer is the same as 78 × 5 with a zero tagged to the end. We can write that zero first, and then multiply as if we were multiplying 5 × 78. (Of course this same shortcut works also if one of the factors is a multiple of 100 or 1000 etc.)

The lesson also includes one word problem involving area and a calculation where we need to consider the order of operations.

See also

The standard algorithm: two-digit multiplication

Math Mammoth Grade 4 curriculum

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