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Before continuing with multiplication it might be wise to review place value.
Because we use a Base10 number system, multiplying by 10 or 100 (or any multiple of 10) is fairly easy.
As the picture below shows, each place has a value 10 times that of the place immediately to the right
If necessary, review place value before proceeding with the multiplication methods below.
Multiply by 10 by moving the numbers one place to the left and adding one zero.  

Multiply by 100 by moving the numbers two places to the left and adding two zeros.  

Quick Way: count the zeros and put them to right of the number being
multiplied.
e.g. 34 x 100 = 3400
Try this multiplication worksheet generator. You can select to multiply by multiples of 10 or multiples of 100 and get limitless questions to print out and practice with.