P varies directly with Q and inversely with R, on the contrary, P varies inversely with R, this means P and R are indirectly proportional. In simple terms it means whatever you do to P, you do the opposite to R. Or P=K*Q/R
18=k*5/2
18=5k/2
5k=36
k=36/5
P=K*Q/R
p=(36/5)(20/10)
p=72/5