The average of 40 numbers is 36. The average of the first 25 numbers is 31 and the average of last 16 numbers is 43. Find the 25th number.

Given, average of 40 numbers = 36

$$\Rightarrow$$ Sum of 40 numbers = 36 x 40 = 1440..................(1)

The average of the first 25 numbers is 31

$$\Rightarrow$$ Sum of first 25 numbers = 31 x 25 = 775

$$\Rightarrow$$ Sum of first 24 numbers + 25th number = 775 ......(2)

The average of last 16 numbers is 43

$$\Rightarrow$$ Sum of last 16 numbers = 43 x 16 = 688 .............(3)

Adding equations (2) and (3),

Sum of first 24 numbers + 25th number + Sum of last 16 numbers = 775 + 688

$$\Rightarrow$$  Sum of first 24 numbers + Sum of last 16 numbers + 25th number = 1463

$$\Rightarrow$$  Sum of 40 numbers + 25th number = 1463

$$\Rightarrow$$  1440 + 25th number = 1463

$$\Rightarrow$$  25th number = 23

Hence, the correct answer is Option B