Sampling and estimation — practice question

The speeds, in $ \mathrm{km\,h^{-1}}$, of $90$ cars as they went past a particular marker on a road were noted, rounded to the nearest $ \mathrm{km\,h^{-1}}$. The findings are arranged in the table below: Speed ($\mathrm{km\,h^{-1}}$): $10\text{-}29, 30\text{-}39, 40\text{-}49, 50\text{-}59, 60\text{-}89$. Frequency: $10, 24, 30, 14, 12$.