Answer:
894.19 mi/hr
Step-by-step explanation:
Total distance of the trip = 3990 mi
Let the speed of the sound be 'x' miles per hour
Now,
Total distance = Speed × Time
Therefore,
According to the question
3,990 mi = [ ( x - 120 ) × 0.1 ] + [ ( x + 410 ) × 3.0 ]
or
3,990 mi = 0.1x - 12 + 3x + 1230
or
0.1x + 3x = 2772
or
3.1x = 2772
or
x = 894.19 mi/hr