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Help-AS statistics question!

Hi,
Can someone please explain what to do for this question.
I've tried it but my answer is way too big.
Question:
The following table shows the results of a survey to find the average daily time, in minutes, that agroup of schoolchildren spent in internet chat rooms.
Time per day/Frequency (t minutes)
(0 ≤ t < 10)= 2
(10 ≤ t < 20) =f
(20 ≤ t < 40) =11
(40 ≤ t < 80) =4
The mean time was calculated to be 27.5 minutes.
(i) Form an equation involving f and hence show that the total number of children in the survey
was 26.

Reply 1

sunnydespair

(2x5 + 15f + 30x11 + 60x4 ) / (2 + f + 11 + 4) = 27.5