Question 639035
Satellite I and satellite II take 20 hours to do the job

They work together for 12 hours

so they finsish 12/20 of the job 
= 3/5 

Balance job is 1- 3/5  = 2/5

2/5 of the job the satellite II does in 24 hours

so in 1 hour it does 2/24*5 of the job = 2/120 = 1/60

Together they can do the job in 20 hours
Satellite II does 1/60 of the job in 1 hour


so Satellite I does 1/20 -1/60 of the job in 1 hour

=2/60 = 1/30 of the job in 1hour.

Satellite takes 60 hours to do the job alone
Satellite I takes 30 hours to do the job alone.


Joshua does 1/8 of the job in 1 hour
David does 1/9 of the job in 1 hour

so together they do 1/8 +1/9 of the job in 1 hour

=17/72  of the job in 1 hour

so they take 72/17 hours to complete the job together.

4.23 hours


speed of boat be x
current = 10 kph

speed with current = x+10
speed against current = x-10
Return trip takes 20 minutes more

time going =time return + 1/3 hours

OR
Time return - time going = 1/3

Time = distance / speed

40/(x-10) -40/(x+10)= 1/3

LCD = 3(x+10)(x-10)

3*40(x+10)-3*40(x-10)= (x+10)(x-10)

120x+1200 -120x+1200=x^2-100

x^2= 2500

x= 25

speed of boat = 25 kph

m.ananth@hotmail.ca