I am given the following probability density function:

$\displaystyle f (x,y) = \frac{2x + 2 - y}{4}$ for 0<x<1 and 0<y<2

$\displaystyle f (x,y) = 0$ otherwise

I am to find P[X + Y > 1].

My attempts at the double integration failed.

The answer begins:

$\displaystyle \int_0^1 \int_{1-x}^2 \frac{2x + 2 - y}{4} dydx$

How did they pick which integral was inner?

How did they choose 1-x as the lower limit on the inner integral?