(i) To draw the histogram, calculate the frequency density for each class interval using the formula:

\(\text{Frequency Density} = \frac{\text{Frequency}}{\text{Class Width}}\)

For \(0 \leq t < 20\): \(\text{FD} = \frac{320}{20} = 16\)

For \(20 \leq t < 40\): \(\text{FD} = \frac{280}{20} = 14\)

For \(40 \leq t < 60\): \(\text{FD} = \frac{220}{20} = 11\)

For \(60 \leq t < 100\): \(\text{FD} = \frac{220}{40} = 5.5\)

For \(100 \leq t < 140\): \(\text{FD} = \frac{100}{40} = 2.5\)

Draw bars with these heights on the histogram.

(ii) To estimate the mean, use the midpoints of each class interval:

\(\text{Mean} = \frac{\sum (\text{Midpoint} \times \text{Frequency})}{\text{Total Frequency}}\)

\(= \frac{(10 \times 320) + (30 \times 280) + (50 \times 220) + (80 \times 220) + (120 \times 100)}{1140}\)

\(= \frac{3200 + 8400 + 11000 + 17600 + 12000}{1140}\)

\(= \frac{52200}{1140}\)

\(= 45.8\)