Question50 students answered a math question. Given the time intervals and frequencies, estimate the mean time taken. Use intervals: $10<t \leqslant 20$, $20<t \leqslant 25$, $25<t \leqslant 30$, $30<t \leqslant 50$, $50<t \leqslant 80$. Frequencies: 2, 8, 12, 16, 12.