Hello,
We write v the speed of the boat in calm water and t the time of the travel.
We obtain the following system of equations:
t(v + 7) = 98
t(v - 7) = 49
So we have :
2 × t(v - 7) = 49 × 2
2t(v - 7) = 98
We can therefore establish the following equation:
2t(v - 7) = t(v + 7)
2tv - 14t = tv + 7t
2tv - tv = 7t + 14t
tv = 21t
v = 21t/t (if t ≠ 0)
v = 21
Thus, the speed is 21 mph
