x = suma initiala de bani
x - 1/4 x - 300 - 2/5 ( x - 1/4 x - 300) - 100 = 2600
3/4 x - 2/5 ( 3/4 x - 300) = 3000
3/4 x - 3/10 x + 120 = 3000
15/20 x - 6/20 x = 2880
9/20 x = 2880 ==> x = 6400 lei
