i love math but i just cant answer it...?

Question

1) a boat can go 9 km downstream in 45 mins. and return upstream in an hour. find the speed of the boat in still water and the speed of the current

2) the total time taken to travel x km at 30 km/h is 2 1/2 hours. the time taken to travel x km at 15 km/h and y km at 45km/h is 4 hour and 20 mins. find x and y

Answer 1

Let X= speed of boat, let y= speed of current

so X-y = 9, since 45 minutes is 3/4 of an hour we know that x + y =12 (9/(3/4)
If you solve this you will get x+10.5 and y=1.5

2) 2.5 hrs*30km/hr= 75 km so this is not solveable at 15km/hr it would take 5 hours to go 75km. No value of y works even if it could go 1,000,000 km/hr the time is already used up.

Answer 2

Notice that the speed downstream in km/hr is 9/(3/4) = 12 km/her. Upstream speed is 9/1 = 9 km/hr.

The speed of the boat downstream is the sum of the boat speed in still water plus the speed of the current. Upstream the speed is boat speed in still water minus water speed downstream.

So if you let x = boat speed in still water and

y = water speed down stream you get

x + y = 12 and x - y = 9.

Solve this system by adding or subtracting equations to get

x = 10 1/2 km/her for the boat speed and

y = 1 1/2 km/hr for the speed of the current.


Analyzie the second problem in the same way.