Fuel efficiency (in km per litre) recorded for 40 scooters at a service camp:
10.5-13.5: 5, 13.5-16.5: 12, 16.5-19.5: 15, 19.5-22.5: 8.
Find the mean fuel efficiency.

Class marks 12, 15, 18, 21 give $\sum f_i{x}_i=60+180+270+168=678$ with $\sum f_i=40$, so the mean is $\frac{678}{40}=16.95$ km/l. Note the class marks are whole numbers even though the limits are not. "15.45" uses the lower boundaries 10.5, 13.5, ... as $x_i$; "18.45" uses the upper boundaries; "169.5" divides by 4, the number of classes.