Answer: 2mph
Explanation:
In relative vector : Considering objects A and B
a) when two objects are moving in the same direction
Vab = Va + Vb
b) when two objects are moving in an opposite direction
Van = Va - Vb
Hence, this rule will be applied.
Speed upstream = ( 33-v ) opposite direction of the current
Downstream = ( 33+v ) same direction with the current
Distance = speed × time
Time upstream = 35/60
Time downstream = 31/60
From the formulae distance = speed × time
Upstream = ( 33-v ) × 35/60 ............ 1
Downstream = ( 33+V ) × 31/60 .............. 2
Equate 1 and 2
33-v) ×35/60 = 33+V) ×31/60
Multiply both side by 60
1155-35v = 1023+31v
1155-1023 = 31+35
132 = 66v
V = 2mph
