Answer:
Peter rents 6 of the rafts seating 3 people, and 7 of the rafts seating 5 people.
Explanation:
Let
denote the number of rafts seating 3 people and
the number of rafts seating 5 people. The total number of rafts is 13, so
. We can also assume that every raft is filled to the brim (it isn't stated explicitly though, but it's probably the intention of the question maker), so
.
It's always a good idea to put these equations under each other:
We can subtract the first equation three times from the second one to obtain
Now, substitute this found value for
into
and we see that
. We are now done: Peter rents 6 of the rafts seating 3 people, and 7 of the rafts seating 5 people.