Simultaneous equation (?) word problem

I'm struggling with this...

"Three types of ship are used to transport goods, type P, Q and R. The ships are the same shape and design, but have different sizes. Their cargo capacity depends on the hold volume. The number of crew required is proportional to the surface area of the deck. Types Q and R taken together have the same length as 2 type Ps. The crew of a type Q is just sufficient to provide crew for two type Ps and a type R.

A fully loaded type Q wants to transfer all of its cargo to smaller Ps and Rs, while minimising the nymber of crew needed for the resultant fleet. How many type Ps and Rs are needed?"

It also gives a hint, saying that for obhects of any shape the surface area is proportional to the square of the object's size and the volume proportional to the cube of its size.

I'd really appreciate any help.

Thank you!