The origin of this problem is the proportion in equal deformation.

For example, 0.4 = 400/1000 0.4 = 40/100, then 400/ 1000 = 40%.

That is 400/ 1000 = 40%, that is, 400 /40% = 1000. So the price of the corresponding share divided by the corresponding percentage is the whole price.

Or understand it this way: if the integer is x, then 400/x = 40%, then x = 400/40% = 1000.

Proportional principle: A/B = C/D, and the deformation is:

A = B * C/D

B = A / (C/D)

C = (A / B )* D

D = C / (A/B)

The key to learning mathematics is to know how the problem comes from, and it is easy to understand when you know it.