A Round Trip of Plane

'x' >> Hours travelled in first part by 105mph. The equation will be:

105x + 115(5-x) = 555 (Speed x time = distance)

so, x=2hrs=120 min

555=105(x/60)+115[5-(x/60) ] x=120

only 120 min can satisfy the condition to near approximation

You can figure out the answer from the options. Take the average of the speeds --> (105+115)/2 = 110 which is very close 111 ( the actual average speed of the plane...555/5) This means that the plane traveled at 115 for a longer time than 105, but only slightly more so. Only 120 minutes satisfies this condition. The other options would jack up the average speed to a larger value.

total distance required for travel is =555miles let the first part complete in 'x' hours and the second part in'y ' hours so the total is x+y=5............(1) speed =distance/time so, 105x+115y=555.............(2) BY SOLVING THESE TWO EQUATIONS YOU WILL REACH THE ANSWER... ie X=2 hours & Y =3 hours ie x=2*60=120m

Let the time for which the plane was travelling at the average speed of $105 mph$ be $'x'$ . Therefore, the time for which the plane was travelling at the average speed of $115 mph$ will be $(5-x)$ .

$105x + 115(5-x) = 555$

Solving this equation for $'x'$ , we get $x = 2 hours$ or $\boxed {120 minutes}.$