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The reverse, in the sense of undoing the effects of "divide by 21" is "multiply by 21".
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21
divide by 21
Divide by 4.
Divide by 622.
-44
Divide 42 by 2 to give you 21.
72 reverse the digits divide by 3 = 24
21 divide into 6450 = 0.0032558139534883722
Divide by 9
Multiply by 36
multiply by 1/38
21*1 = 21