fie a>b, numarul timbrelordin fiecare clasor
a+b=270
a-10a/100=b+10a/100
a+b=270
a-a/10=b+a/10
a+b=270
a-2a/10=b
a+b=270
a-a/5=b
a+b=270
b=4a/5
a+4a/5=270
9a/5=270
a/5=30
a=150
atunci b= 270-a=270-150=120
verificare 10% din150=15
150-15=120+15 adevarat , problema este bine rezolvata