Question 1019449
Call the time through the mountains, t.
Since 3 hours, 48 minutes is 3 4/5 (or 3.8) hours, the time on the plains must be 3.8 - t.
We can set this up, for rt = d, as
90(3.8 - t) + 37.5t = 300
342 - 90t + 37.5t = 300
-52.5t = -42
t = .8 hours = 48 minutes