I'm assuming your teacher wants you to calculate the speed
1/2 = 0.5
3 & 1/2 = 3 + 1/2 = 3 + 0.5 = 3.5
If a plane flies 840 miles in 3.5 hours, then,
speed = distance/time
speed = (840 mi)/(3.5 hr)
speed = (840/3.5) mph
speed = 240 mph