7.  The mean of the following distribution is 50.

x	f
10 30 50 70 90	17 5a + 3 32 7a - 11 19

Find the value of a hence the frequencies of 30 and 70.

what 7^th one?




 

250=17+5a+3+32+7a-11+19
250=60-12a
190=12a
a=190/12
a=15.83

good

50=640a+3100/12a+70
that is (12a+70)50=640a+3100
600a+3500=640a+3100
640a-600a=3500-3100
40a=400
a=10
Frequency of 30=5a+13
                             5*10+13
                               =63
Frequency of 70=7a-11
​                            =70-11
                            =59
​