Answer :
Well, let's gather the information. And set the equations.
p+q+r=80
p/r =3/5 this is the same as 3r=5p p=3/5r or r= 5/3p
r/q=10/4 this is the same as 10q=4r so or r=5/2q q=4/10r simplifying q =2/5r
Let's plugg it in the first equation:
p+q+r=80
p+q+5/2q=80
p+2/5r+5/2q=80
p+2/5r+5/2q=80
p+2/5(5/3)p+5/2q=80
p+2/3p+5/2q=80
p+2/3p=80-5/2q
5/3p=80-5/2q x 3
5p=240-15/2q :5
p=48-3/2q
(48-3/2q)+q+5/2q=80
48-3/2q+q+5/2q=80
48-2/2q+q=80
48+2q=80
2q=80-48
2q=32
q=16
p=48-3/2(16)
p=48-3/2
p=24
pq=24*32=384
The answer is C) 384