48 mph
The answer is not 50 mph, as the time it takes to get there is not going to be 1/2 of the time 40 mph, and 1/2 of the time 60 mph, rather 40 mph will take up 3/5 of the trip and 60 mph will take up 2/5 of the trip (as when you go at 60 mph, you will reach the destination sooner compared to 40 mph).
120/40 = 3
120/60 = 2
Because of speed differences, this means that the answer is 48
40 * 3/5 = 24
60 * 2/5 = 24
24 + 24 = 48
I know it seems confusing but I hope you understand!