Answer:
23.1 hours
Step-by-step explanation:
the mean is calculated as
mean = ∑ fx ÷ ∑ f
where x is the midpoint of the time intervals
0 < h ≤ 10 ⇒ x = 5
10 < h ≤ 20 ⇒ x = 15
20 < h ≤ 30 ⇒ x = 25
30 < h ≤ 40 ⇒ x = 35
then
∑ fx = (5 × 5) + (15 × 15) + (50 × 25) + (10 × 35)
= 25 + 225 + 1250 + 350
= 1850
∑ f = 5 + 15 + 50 + 10 = 80
then
mean = 1850 ÷ 80 ≈ 23.1 hours (to 1 dp )
