Step-by-step explanation:
if I understand your comment correctly, then you need the mean value of the lifetime of these light bulbs.
1. get the midpoint of each interval.
2. multiply each frequency with its midpoint.
3. sum all frequencies.
4. sum all multiplication results if 2. (of frequencies and midpoints).
5. divide 4. by 3.
lifetime midpoints :
(25 + 50)/2 = 75/2 = 37.5
(50 + 100)/2 = 150/2 = 75
(100 + 150)/2 = 250/2 = 125
(150 + 200)/2 = 350/2 = 175
(200 + 250)/2 = 450/2 = 225
(250 + 300)/2 = 550/2 = 275
(300 + 350)/2 = 650/2 = 325
multiply frequencies with their midpoints :
37.5 × 47 = 1762.5
75 × 86 = 6450
125 × 45 = 5625
175 × 36 = 6300
225 × 0 = 0
275 × 37 = 10175
325 × 50 = 16250
sum all frequencies
47+86+45+36+0+37+50 = 301
sum all multiplication results of 2.
1762.5+6450+5625+6300+0+10175+16250 = 46,562.5
divide 4. by 3.
the mean value is
46,562.5 / 301 = 154.692691... hours ≈
≈ 154.69 hours
