cooking home work

we got this odd homework and im having a hard time tring to figure it out by myself
so it goes like:
from some ammount of sugar, 15 kg-s were used up, then the 1/5 of the remaing sugar is also used , but there is still as much sugar left that were used up untill now +3 kg-s
so how much sugar did we have at firts ???

thx in advance

Consult Al Jabr

If x is the total amount.  Then u (The amount used) is equal to 15 + .2(x-15).  So the amount left (L) is x - u.  Also, L = u + 3.

u = 15 + .2(x-15)
L = x - u
L = u + 3

x - (15 + .2(x-15)) = 15 + .2(x-15) + 3
x - 15 - .2(x-15) = 15 + .2(x-15) + 3
x = 30 + .4(x-15) + 3
x = 33 + .4(x-15)
x = 33 + .4x - 6
x = 27 + .4x
.6x = 27
x = 45 kg