M=mama
I=Irina
M+I=30 => I=30-M
M= Ix5 => M=(30-M) x5 (am inlocuit I cu cat era I-ul mai sus)
M=30x5-5xM => M=150-5xM| +5xM => 6xM=150 => M=150/6=25
Si cum stiam mai sus, M+I=30 => I=30-M => I=30-25=5
