A freight train covered 120 miles at a certain speed. Had the train been able to travel 10 mph faster, the trip would have been 2 hours shorter. How fast did the train go?
That other "tutor" is only showing his ignorance of algebra.
Make this chart:
Distance Rate Time
Trip that happened 120 x t
Trip that didn't 120 x+10 t-2
Use DISTANCE = RATE × TIME, we get this system
from the chart:
120 = xt
120 = (x+10)(t-2)
Since they're both equal to 120, set them equal
xt = (x+10)(t-2)
Multiply out the right side
xt = xt-2x+10t-20
Subtract xt from both sides:
0 = -2x+10t-20
2x = 10t-20
Divide through by 2
x = 5t-10
Substitute 5t-10 for x in
120 = xt
120 = (5t-10)t
120 = 5t²-10t
-5t²+10t+120 = 0
Divide through by -5
t²-2t-24 = 0
(t-6)(t+4) = 0
t-6 = 0; t+4 = 0
t = 6; t = -4 (ignore)
x = 5t-10
x = 5(6)-10
x = 30-10
x = 20
The train went 20 mph
Edwin