math
posted by Mat on .
1.A father can do a job in a certain number of hours. His son takes twice as long to do the job. Working together, they can do the job in 6 hours. How many hours does it take the father to do the job alone?
A.9
B.18
C.12
D.10
3.David is 5 years older than Paul. In 5 years, David will be twice as old as Paul is now. How old is David now?
A.15
B.10
C.25
D.20
this is what i have so far:
D=p+5
2D+p(i'm not sure if this is right)

1.
combined rate=1/F +.5/F=1.5/F=1/6
f=9 hrs
D5=P
D+5=2P
subtract second equation from first
10=P P is 10 now, so D is 15 
job/rate = time > rate = job/time
let the fathers rate = job/x
let the son's rate be job/2x
combined rate =job/x + job/2x
= 3job/(2x)
combined time = job รท(3job/2x)
= job (2x/3job)
= 2x/3
but 2x/3 = 6
2x = 18
x = 9
It would take 9 hrs for the father to work alone
3.
Paul's age  x
David's age  x+5
in 5 years:
David > x+10
x+10 = 2x
x = 10
Paul is now 10 and David is now 15