UPSC CAPF Assistant Commandant Model Paper 4

GIVEN:
Distance between thief and policeman = 400 m
Speed of = 30.8 km/h
CONCEPT:
Speed, distance and time
FORMULA USED:
Distance = Speed × Time
CALCULATION:
Let speed of thief = ‘x’ km/h
Relative speed of thief and policeman = 30.8 – x
Distance between thief and policeman when thief is noticed = 0.4 + x × (15 / 60) = 0.25x + 0.4
Now,
0.25x + 0.4 = (30.8 – x) × (30 / 60)
⇒ 0.5x + 0.8 = 30.8 – x
⇒ 1.5x = 30
⇒ x = 20
∴ Speed of thief = x = 20 km/h