Let v m/s be the swimming speed of man and u m/s be the speed of river.
for minimum time t to cross the river of width d, we have t = d / v ..............(1)
By substituting the values in eqn.(1), we have 25 = 100 / v or v = 4 m/s
drift distance x for minimum time crossing is given by, x = u×(d / v) ....................(2)
By substituting the values in eqn.(2), we have 75 = u×(100/4) or u = 3 m/s
time taken t1 for 70 m downstream = 70 / (4+3) = 10 s
time taken t2 for 70 m upstream = 70 / (4-3) = 70 s
hence ( t2 / t1 ) = 70/10 = 7