Since this is a most question, we should start with choice E and move backwards.

Drew a chart like this:

Laticia

(Assuming answer is E - 8.5miles)

3.5 miles ---- 6 min mile - 21 min

5 miles ------- 8 min mile -- 40 min

Total = 61 min

Johanna

(Assuming answer is 8.5 miles)

8.5 miles --- 7 min mile ---- 59.5 min

since 59.5 < 61 we must choose another answer

Assuming answer is (7 miles)

Laticia

3.5 miles --- 6 min mile --- 21 min

3.5 miles ---- 8 min mile --- 28 min

Total = 49 min

Johanna

7 miles --- 7 min mile --- 49

This is the answer : D

I think GMAT doesnt test us on our formulas or even remotely medium formulaes. It usually boils down to simple addition and subtraction. They just want u to set up the addition and subtraction. Dont be confused by setting up equations. Use POE and plugging in.