1) Fie a,b,c partile si avem:
{a,b,c} dp {4,5,11}
a=4k
b=5k
c=11k
a+b+c=120
4k+5k+11k=120
20k=120
k=6
a=4*6=24
b=5*6=30
c=11*6=66
2)
Fie x suma initiala.
80% din x=80x/100=4x/5 -prima suma cheltuita
20% din 4x/5=20/100*4x/5=1/5*4x/5=4x/(5*5)=4x/25 - a doua suma cheltuita
4x/5+4x/25=(5*4x+4x)/25=24x/25 - suma cheltuita in total
x-24x/25=x/25 - suma ramasa (cea care valoreaza 15 lei)
x/25=15
x=25*15
x=375 lei
Cassian a avut initial 375 lei.