Proportion Concept

When we say that P varies directly with Q, we mean that when P increases, Q increases proportionally. When P decreases, Q decreases proportionally. The word proportionally means that if P doubles, Q also doubles. If P becomes 1/3, Q also becomes 1/3
Mathematically, we write this as P α Q or P = kQ

e.g. The cost of renting a hall varies directly with the time for which it is rented. This means Cost = k*Time where k is a constant or Cost/Time = k, a constant.
If you rent the hall for 1 hr, you pay $100. How much will you pay if you rent it for 4 hrs?
If C = 100, T = 1, then k = 100/1 = 100
Now, if T = 4, what is C? C = kT, therefore, C = 100*4 = $400

There is also inverse variation. P varies inversely with Q means when P increases, Q decreasea proportionally and vice versa.
Mathematically, P α 1/Q or PQ = k.

e.g. The cost of a standard gold coin is inversely proportional to the amount of impurities in it. So Cost * Impurities = k.
If a standard gold coin has 4 gm impurities, its cost is $400. What is the amount of impurities in a $ 1200 coin.
So, 4 * 400 = k = x * 1200
x = 4/3 gm

This is how you deal with variation. Your questions states P varies as product of Q and R. (directly is implied here)
P = k*Q*R

\(\frac{P}{QR} = k = \frac{3P}{(Q/2)R'}\)
P and Q get canceled and you get R' = 6R
So R increases by 5R.