I think that the answer should be around 2.55
The factored form of 20x+35y would be 5(4x+7y). If you use the distributive property of multiplication, you can see that the expression has not changed.
Total miles:350+150=500
Total hours:4+3=7
Avg speed:500/7=71.429 mph
X+y = 2
(2*2 + 2*2)
(4 + 2*2)
(4 + 4)
= 8