Sampling and estimation — practice question

The table below summarises the heights, to the nearest metre, of $134$ office buildings in a particular city; the height groups are $21\text{-}40$, $41\text{-}45$, $46\text{-}50$, $51\text{-}60$, $61\text{-}80$ metres, and the corresponding frequencies are $18, 15, 21, 52,$ and $28$.