Let H = rate of Hank in still water.
Let C = rate of the current.
The rate downstream is: H + C
The rate upstream is: H - C
Distance = rate x time
downstream: 1 = (H + C) · 13 ---> 1 = 13H + 13C ---> x 24 ---> 24 = 312H + 312C
upstream: 1 = (H - C) · 24 ---> 1 = 24H - 24C ---> x 13 ---> 13 = 312H - 312C
Adding down the columns: 37 = 624H ---> H = 37/624
24 = 312H + 312C ---> 24 = 312H + 312C
13 = 312H - 312C ---> x -1 ---> -13 = -312H + 313C
Adding down the columns: 11 = 624C ---> C = 11/624
Both of the rates are in terms of miles per minute.