x - szukany procent
c - cena sukni (początkowa)
c - c*p% - cena po obniżce
c = c - c*p% + (c - c*p%)*x%
c = c(1 - p% + (1 - p%)*x%)
1 = 1 - p% + x% - p%*x% /*10000
10000 = 10000 - 100p + 100x - px
100p = 100x - px
100p = (100 - p)x
x = 100p/(100 - p)
