Answer:
They spent $15.
Explanation:
It's given that Jane bought x cookies, so 3x.
John spent $1 on each of the x cookies he got, and also an extra $10, so x+10.
3x=x+10
2x=10
x=5
So they got 5 cookies each.
5 cookies x $3= $15 (check with $5(1)+$10= $5+$10= $15 )