Question 1121921
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Two numbers x and y are multiplied to get a constant, nza.<br>
If one of the two numbers is halved, in order to keep the constant product the other number needs to be doubled.<br>
"Doubling and halving" is a common practice in mental arithmetic.  For example, to multiply 12 times 35 you can double the 35 to 70 and halve the 12 to 6 and get 6 times 70.  With the trailing 0 in one of the numbers the multiplication is easy:<br>
12*35 = 6*70 = 420