Maths word problem

Question

30 children are going on a trip. It cost £5 including lunch. Some children take their own packed lunch, they pay only £3. The 30 children pay a total of £110.How many children are taking their own packed lunch?

Answer 1

Let L = # children bringing their own lunch; they pay £3

     N = # children not bringing their own lunch; they pay £5

 

From the first sentence, 

L + N = 30

3L + 5N = 110

 

We need to find L

We can use substitution

L+N=30

N = 30-L

Substitute the expression for N, above into the equation 3L + 5N = 110

3L + 5(30-L) = 110

3L + 150 - 5L = 110

150-2L=110

-2L = -40

L = 20

20 Children pack their own lunch.