The motorcyclist expected to travel 120 km in a certain time. An hour after departure, he had to make a 15 mn stop.

The motorcyclist expected to travel 120 km in a certain time. An hour after departure, he had to make a 15 mn stop. To arrive at their destination on time, the motorcyclist increased his initial speed by 1.2 times. What is the initial speed.

Let the initial speed be x km / h, and after stopping – 1.2 km / h. Let’s make an equation based on the formula t = S: V:
120: X = (120: 1.2X) + 0.25, where 0.25 is a 15 minute stop.
144 = 120 + 0.3X,
0.3X = 144 – 120,
0.3X = 24,
X = 80 km / h – the initial speed of the motorcyclist.
Answer: 80 km / h initial speed.