Notam suma cu S, deci: S=suma
PRIMA ZI:
•cheltuie: (3/5)×S
•raman: S-(3/5)S = (5/5)S-(3/5)S = (5/5-3/5)S = [(5-3)/5]S = (2/3)S
A DOUA ZI:
•cheltuie: (7/10)×(2/3)×S = (14/30)S =(7/15)S
•raman: (2/3)S-(7/15)S = (2/3-7/15)S = (10/15-7/15)S = [(10-7)/15]S = (3/15)S = (1/5)S
A TREIA ZI:
•cheltuie: (1/5)S=40 lei
(1/5)×S=40
1×S= 40×5
S=200 lei