Quick question about partial pressures

Okay, so this is the question:

A gaseous mixture of Radon and Krypton contains 3 moles of Radon for every 2 moles of Krypton. The partial pressure of Krypton is?

I know the answer is 2/5 of the total pressure - but, how do I get that answer?

Partial pressure of krypton = moles of krypton / total amount of moles present in the system
So, partial pressure = 2/(2+3) = 2/5

And the partial pressure of radon would be: 3/(2+3) = 3/5