F.3 math

1.The mean of a，b，c is 6；that of c，d，e is 7 andthat of a，b，c，d，e is 6.4.
Find c
Sol
a+b+c=3*6=18
c+d+e=3*7=21
a+b+c+d+e=5*6.4=32
c=(a+b+c)+(c+d+e)－(a+b+c+d+e)
=18+21－32
=7

2.The mean of a group of numbers is 5. If 13 is added as a datum, the meanbecomes
6. Find the original number of data.
Sol
Set the original number of data. is x
(5x+13)/(x+1)=6
5x+13=6x+6
x=7