A runner can run one mile in 7 minutes, but then for each mile after that her pace slows by 30 seconds per mile (i.e. she runs the second mile in 7:30, the third in 8 minutes etc). After how many miles will her overall average pace be 9 minutes per mile? 5 6 7 8 9

A runner can run one mile in 7 minutes, but then for each mile after that her pace slows by 30 seconds per mile (i.e. she runs the second mile in 7:30, the third in 8 minutes etc).

After how many miles will her overall average pace be 9 minutes per mile?