Given: Average of the five numbers = -5 Sum of the first three numbers = 15 Formula Used: Average of observations = Sum of all the numbers in the observations/Number of observations Calculation: Let the five numbers be a, b, c, d, and e. So, we have: a + b + c + d + e = 5 × -5 = -25 ----(i) Also, a + b + c = 15 ----(ii) On substituting the value of equation (i) into equation (ii), we get: 15 + d + e = -25 ⇒ d + e = -40 ⇒ Average of d and e = -40/2 = -20 ∴ The average of the last two numbers is -20