m=mary's age
c=chris's age
m is 4 younger than 3 times chris
m=-4+3c
m=3c-4
mary's age 3 years ago (m-3) is equal to chris age in 9 years (c+9), find c and m
m-3=c+9
we got
m=3c-4
sub 3c-4 for m in other eqution
3c-4-3=c+9
3c-7=c+9
minus c both sides
2c-7=9
add 7 to both sides
2c=16
divide both sides by 2
c=8
sub back
m=3c-4
m=3(8)-4
m=24-4
m=20
mary=20
chris=8
