Question

A Boat goes upstream 42 km and downstream 72 km in 13 has it also goes upstream 36 km and downstream 48 km in 10 hours find the speed of the boat and stream

Answer 1

Hi ,

Let the speed of the boat

in still water = x kmph

Speed of the stream = y kmph

i ) relative speed of the

boat in downstream

= ( x + y ) kmph

Distance travelled = d1 = 36

Time = t1 hr

t1 = d1 / s1

t1 = 36/ ( x + y )

ii) relative speed of the boat

in upstream = ( x - y ) kmph

Distancw = d2 = 32 km

Time = t2

t2 = 32/ ( x - y )

Therefore ,

Total time = 7 hr

t1 + t2 = 7hr

36 / ( x + y ) + 32/ ( x - y ) = 7 ----( 1 )

iii) second time ,

Relativespeed of the boat in

downstream = ( x + y ) kmph

d3 = 48 km

Time = t3

t3 = 48/ ( x + y )

iv ) in upstream

Relative speed of the boat = ( x - y )

kmph

time = t4 hr

d4 = 40km

t4 = 40/ ( x - y )

Total time = 9 hr

48 / ( x + y ) + 40/ ( x - y ) = 9 ---( 2 )

Let 1 / ( x + y ) = a ,

1 / ( x - y ) = b

Then rewrite ( 1 ) and ( 2 ) we get

36 a + 32 b = 7 -----( 3 )

48a + 40b = 9 ------( 4 )

Multiply ( 4 ) with 4 and equation ( 3 ) with 5 and

192a + 160b = 36 ---( 5 )

180a + 160b = 35 -----( 6 )

Subtract ( 6 ) from ( 5 )

we get

a = 1/ 12

put a = 1/ 12 in ( 3 )

we get ,

b = 1/ 8

Now 1/ ( x + y ) = 1/ 12

1/ ( x - y ) = 1/ 8

Therefore ,

x + y = 12 ----( 7 )

x - y = 8 ----- ( 8 )

add ( 7 ) and ( 8 )

2x = 20

x = 10

put x = 10 in ( 7 ) we get

y = 2

Speed of the boat in

still water = x = 10 kmph

speed of the stream

= y = 2kmph

I hope this helps you.

Answer 2

hope it helps!
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