Aptitude - Boats and Streams - Discussion
Discussion Forum : Boats and Streams - General Questions (Q.No. 8)
8.
A boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream. If the speed of the boat in still water is 10 mph, the speed of the stream is:
Answer: Option
Explanation:
Let the speed of the stream x mph. Then,
Speed downstream = (10 + x) mph,
Speed upstream = (10 - x) mph.
 |
36 | - | 36 | = | 90 |
| (10 - x) | (10 + x) | 60 |
72x x 60 = 90 (100 - x2)
x2 + 48x - 100 = 0
(x+ 50)(x - 2) = 0
x = 2 mph.
Discussion:
85 comments Page 9 of 9.
Abc said:
1 decade ago
Because the unit is in hours.
Karthik said:
1 decade ago
Why we should convert min to hour anybody?
Bhimu said:
1 decade ago
60 is to convert min to hour.
Gopal said:
1 decade ago
Hi there,
I dont understand from where the 60 comes. is it a concept or derived from theose data? pls explain.
You should multiply (10 - x) and (10 + x) as to take LCM, so that you can substract the numeraters i.e. 36-36.
You knew that (a+b) x (a-b) = (a2 - b2) , (NOTE: read it as a square minus b square.)
And in order to multiply the denominators you got to multiply them with the numerators also; so, you get 36(10+x) - 36 (10-x), this is divided by (100 - x2 -i.e.square)
But what i do not understand is from where does that 60 come?
Anyone?
I dont understand from where the 60 comes. is it a concept or derived from theose data? pls explain.
You should multiply (10 - x) and (10 + x) as to take LCM, so that you can substract the numeraters i.e. 36-36.
You knew that (a+b) x (a-b) = (a2 - b2) , (NOTE: read it as a square minus b square.)
And in order to multiply the denominators you got to multiply them with the numerators also; so, you get 36(10+x) - 36 (10-x), this is divided by (100 - x2 -i.e.square)
But what i do not understand is from where does that 60 come?
Anyone?
Ram said:
1 decade ago
72x x 60 = 90 (100 - x2)
x2 + 48x + 100 = 0
How this step comes? pls tell me.
x2 + 48x + 100 = 0
How this step comes? pls tell me.
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