As a block of concrete is put under increasing pressure, engineers measure the pressure X at which the first fracture appears and the pressure Y at which the second fracture appears. X and Y are measured in tons per square inch and have joint density function fXY(x, y) = { 24x(1 - y) for 0 < x < y < 1
0 otherwise.
Calculate the average pressure (in tons per square inch) at which the second fracture appears, given that the first fracture appears at 1/3 ton per square inch.