Suma initiala = a
Suma cheltuita in prima zi = (2/3) x a + 15
Suma cheltuita in a doua zi = 40% {a - [(2/3) x a + 15]} = (40/100) x [a - (2/3)a + 15] = (40/100) x [(1/3)a + 15]
Restul ramas = 27 RON
a - [(2/3)a + 15] - (40/100) x [(1/3)a + 15] = 27
(1/3)a - 15 - (40/300)a + 600/100 = 27
(1/3)a - 15 - (2/15)a + 6 = 27
(1/3)a - (2/15)a = 27+15-6 = 36
(5/15)a - (2/15)a = 36
(3/15)a = 36
(1/5)a = 36
a = 36 x 5 = 180
Deci suma initiala pe care a avut-o Stefan a fost de 180 RON