Question

The average of one group of numbers is 4. A second group contains twice as many numbers and has an average of 10. What is the average of both groups of numbers? 

Answer 1

so given let s = sum
so average = 4 of s  is s/x =4 
 s = 4x 
 similarly    s₁ = sum
so 2nd average = 10 of s  is s/2x = 10  
 s₁ = 20x
so average of the total numbers is 
     
so total sum of the numbers = 20x + 4x =24x
so total numbers = x+2x = 3x
average = total sum/total numbers
so
24x/3x = 24/3

so the average is 8