Minimum night temperature (in °C) recorded on 30 nights at a high-altitude weather post:
(-6)-(-3): 4, (-3)-0: 7, 0-3: 9, 3-6: 6, 6-9: 4.
Find the mean minimum temperature.

Class marks -4.5, -1.5, 1.5, 4.5, 7.5. $\sum f_i{x}_i=-18-10.5+13.5+27+30=42$ and $\sum f_i=30$, so the mean is $\frac{42}{30}=1.4°C$. Negative class marks are handled exactly like positive ones. "3.3" drops the minus signs before adding; "-0.1" uses the lower limits -6, -3, 0, 3, 6 as $x_i$; "8.4" divides by 5, the number of classes.