A taxi company charges a flat fee plus an amount per mile. A journey of 10 miles costs £7 and a journey of 15 miles costs £9. What would you pay for a journey of 8 miles?
7 = x + 10y
8 = x + 15y
1 = 5y
y =1/5 x = 5
5 + 1/5 *8 = your answer