How does moving the place of a digit change its value

In the familiar decimal number system that we use every day, moving the place
of a digit changes its value by a power of 10 .

In a system of numbers to any other base, moving the place of a digit changes
its value by a power of whatever the base of that number system is.

The correct answer would be "convenience"

On monday,mrs.wise bought 3 lbs of apples at $4per lb= 4*3=$12on tuesday ,20%off and she buy 5 lbs of apples = $4-20%=3.20*5lb=$16example: other wise when she buy 5lbs on monday she have to spend 4*5=20