A bicycle wheel has a radius of 42 cm. It makes 40 revolutions in 25 seconds. What is its speed (in km/h, up to one decimal place)?

15.2

Total distance covered = Number of revolutions × Circumference of wheel

Circumference of Wheel = 2Πr = 2 × \(\frac{22}{7}\) × 42 = 264 cm

Total distance covered = 40 × 264 = 10560 cm = 105.60 m

Speed = \(\frac{Total distance }{Time }\)

= \(\frac{105.60}{25}\) = 4.224 m/sec

4.224 m/sec  = 4.224 × \(\frac{18}{5}\)km/h = 15.2064 km/h

= 15.2 km/h  ( approx)