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20 Nov 2025, 23:42
Kudos
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Let t be the time required for the downstream journey.
Set up two speed-distance-time equations (one for each part of the journey) as follows:
Downstream speed = v+3 = 90/t
Upstream speed = v-3 = 90/(t+0.5)
We have two variables and two equations. Solve them as a pair of simultaneous equations.
Combine them in a way that helps us to eliminate v and get a value for t.
Subtract the second equation from the first.
This gives us
(v+3) - (v-3) = 90/t - 90/(t+0.5)
6 = 90/t - 90/(t+0.5)
Divide both sides by 90:
1/15 = 1/t - 1/(t+0.5)
This will result in a quadratic equation.
To avoid doing the math, try plugging in answer choices.
Start with the middle one, option C, where t = 2.3.
1/15 = 1/2.3 - 1/2.8
It’s quite clear just by looking at the numbers that this will not work.
Similarly, just by looking at the numbers in the other answer choices, we can see that neither will options B, D, and E.
The only one that will work is option A.
Set up two speed-distance-time equations (one for each part of the journey) as follows:
Downstream speed = v+3 = 90/t
Upstream speed = v-3 = 90/(t+0.5)
We have two variables and two equations. Solve them as a pair of simultaneous equations.
Combine them in a way that helps us to eliminate v and get a value for t.
Subtract the second equation from the first.
This gives us
(v+3) - (v-3) = 90/t - 90/(t+0.5)
6 = 90/t - 90/(t+0.5)
Divide both sides by 90:
1/15 = 1/t - 1/(t+0.5)
This will result in a quadratic equation.
To avoid doing the math, try plugging in answer choices.
Start with the middle one, option C, where t = 2.3.
1/15 = 1/2.3 - 1/2.8
It’s quite clear just by looking at the numbers that this will not work.
Similarly, just by looking at the numbers in the other answer choices, we can see that neither will options B, D, and E.
The only one that will work is option A.
11 Apr 2026, 19:40
Kudos
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I ended up framing the equation by including river`s speed as r in it, only to realize that this wasn`t needed at all.
I wasted a lot of time with that dirty equation.
Can someone please tell me why isn`t the speed of river not considered ? Or is there something to be told for that like running water or still water ?
I wasted a lot of time with that dirty equation.
Can someone please tell me why isn`t the speed of river not considered ? Or is there something to be told for that like running water or still water ?
12 Apr 2026, 00:21
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anushree01
You do not need the river’s speed separately because its effect is already built into the two actual speeds, v - 3 upstream and v + 3 downstream.
