Question

A boat makes a 120-mile trip downstream in 3 hours but makes the return trip in 4 hours.

What is the rate of the current?

3 mph
4 mph
5 mph

I need help setting up the problem

Answer 1

First we take out the speed of both upstream and downstream

Answer 2

Speed = Distance/Time
So Downstream the speed is 120/3=40
40mph

Answer 3

and upstream is 120/4

Answer 4

=30

Answer 5

which is 30... but how do i find the current speed though?

Answer 6

the speed of the current will be 40-30=10

Answer 7

ooh i see now, thanks

Answer 8

but i am getting speed of boat as 35 mph and speed of current as 5 mph....:\

Answer 9

@Anas.P ,u got speed of (boat + current) as 40 mph and speed of (boat - current)=30 mph.....isn't it? which makes current speed as 5mph....

Answer 10

how does it make 5?

Answer 11

too easy to use a calcy....
downstream means current will assist boat so total speed = their sum
up stream means current will resist boat so total speed = their difference
as distance covered is same we can equate product of time and speed
3*(boat+current) = 4* (boat - current) = 120
which when easily solved gives, boat = 35mph , current = 5 mph

Answer 12

u got this or should i use variables x and y and solve again with equations?? @willow

Answer 13

@hartnn thanks a lot, sorry i missed it.
But can you also use variables to make it more clear..

Answer 14

i think i understand, but can you use vairables?

Answer 15

sure, let x be the speed of boat,let y be the speed of current
Downstream equation (speed *time = dist )
(x+y)*3=120
(x+y)=40 ---> Equation 1
upstream equation (speed *time = dist )
(x-y)*4=120
(x-y)=30 ---> Equation 2
now from eq 2,x=30+y
this x i will put in eq 1
so 30+y+y=40
so 2y=40-30
so y=10/2
so speed of current = 5mph
okay? :)

Answer 16

Cool thanks..