The first 10 minutes the body moved 6 m / s, the next 1 hour the body moved 72 km / h, the remaining time the body moved 8 m / s.

The first 10 minutes the body moved 6 m / s, the next 1 hour the body moved 72 km / h, the remaining time the body moved 8 m / s. Determine the last period of time if the average speed is 54 km / h.

10 minutes – 6m / s
1 hour = 60 minutes – 72 km / h = (72 * 1000) / 3600 = 20 m / s
x min – 8 m / s.
Vav = 54km / h = (54 * 1000) / 3600 = 15m / s
First, let’s write the formula for the average speed
Vcp = \ frac {S_1 + S_2 + S_3} {t_1 + t_2 + t_3}
t1 = 10 min
S1 = 10 * 6 = 60 m
t2 = 60 min
S2 = 60 * 20 = 1200m
t3 = x min
S3 = 8x
Vcp = 15 m / s
Let’s substitute the data
15 = \ frac {60 + 1200 + 8x} {10 + 60 + x}
15 (70 + x) = 1260 + 8x
15 * 70 + 15x = 1260 + 8x
15x-8x = 1260-1050
7x = 210
x = 30 min
Answer: 30 minutes