D = 90 m
S= (v-3) m/hr
Let T = t
S=(v+3) m/hr [let this be statement 1]
T = t-1/2 = (2t-1)/2 [let this be statement 2]
Since D is the same upstream & downstream:
=> t(v-3) = (v+3)(2t-1)/2
=> 2tv-6t = 2tv+6t-v-3
=> v+3=12t or t=(v+3)/12 [let this be statement 3]
Using the value of t from statement 3 in statement 2 we get T=(V-3)/12
D= S * T
=> 90= (v+3)(v-3)/12
=>v=33 [let this be statement 4]
Therefore, when going downstream:
S = v+3 = 33+3 = 36 m/hr
T = 90/36 = 2.5 hrs.
Not sure if the above is correct. Even if it is, I'm sure there's a much simpler way to solve this.
Your approach is perfect and answer is absolutely correct. But what "I" or "we" need is a fast approach to be able to solve it within 2 minutes!
Anyone with some fast approach?