Out of four numbers the average of the first three is 16 and that of the last three is 15. If the last number is 20 then the first number is
let the four numbers be a,b,c,d
Using formula average = $$\frac{SumofElements}{NumberofElements}$$
Now, the average of first 3 numbers = 16
=> Sum of first 3 numbers=a+b+c = 16 × 3 = 48.......... (1)
Also, average of last three numbers = 15
=> Sum of last three numbers = b+c+d = 15 × 3 = 45......(2)
Subtracting equation (2) from (1)
=> a - d = 3
Since, d = 20
=> a - 20 = 3
=> a = 23
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