20 children go for a picnic in a bus. They are to sit 2 or 3 to a seat. There are 7 seats. How many seats will have 3 children seated on them?

Answer 1

seats with 2 on them --- x
seats with 3 on them --- y

x+y = 7 --- y = 7-x

2x + 3y = 20
2x + 3(7-x) = 20
2x + 21 - 3x = 20
-x = -1
x = 1

1 seat with 2 kids, 6 seats with 3 kids

(totally illegal where I live)