The mean of 10 numbers is 12.5; the mean of the first six is 15 and the last five is 10. The sixth number is
Options:
A . 15
B . 12
C . 18
D . none of these
Answer: Option A : A Let the mean of the last four be A2. Then by the formula for combined mean, 12.5=6×15+4×A26+4;or 125=90+4A2; ∴A2=354 Let the sixth number = x; then taking the sixth number as a collection, the combined mean of this collection and the collection of the last five is 10, by question. ∴ By definition of combined mean 10=1×x+4×3541+4;50=x+35;∴x=15 ∴ sixth number = 15
