x - running speed
10x - speed on bike
v = s/t
s - distance
t - time
t = s/v
s₁ = 10 km
s₂ = 120 km
\(\frac{10}{x} + \frac{120}{10x} = 4\\\\\frac{10}{x} + \frac{12}{x} = 4\ \ //*x\\\\10 + 12 = 4x\\\\4x = 22\ \ //:4\\\\x =\frac{22}{4} = 5,5\ km/h\)
Solution:
Running speed = 5,5 km/h
