a - 5/13 x a - 5/8 × ( a - 5/13 x a) = 90
a - 5/13 x a - 5/8 x a + 5/8 × 5/13 x a= 90
a - 5/13 x a - 5/8 x a + 25 /104 x a = 90 / x104
104a - 5x8 xa - 5×13 × a +25 × a = 90×104
104a - 40 x a- 65×a +25 x a = 90x104
-1 x a + 25 x a = 9360
24 x a = 9360
a = 9360 :24
a=390 kg. zahar
a = cantitatea de zahar
x = inmultit