Option 3 : 80
GIVEN:
Average of first 3 numbers = 60
Average of 2nd, 3rd and 4th number = 20
Average of 3rd, 4th and 5th number = 30
Average of last three numbers = 40
FORMULA USED:
Average = Sum of the number / Total number
CALCULATION:
Let the six numbers be a, b, c, d, e, f.
a + b + c = 3 × 60 =180 ----(1)
b + c + d = 3 × 20 = 60 ----(2)
c + d + e = 3 × 30 = 90 ----(3)
d + e + f = 3 × 40 = 120 ----(4)
Sum of first, fifth and sixth number may be obtained by (1) + (4) - (2) = a + b + c + d + e + f - (b + c + d) = 180 + 120 - 60 = 240
∴ Average of a, e, and f = 240/3 = 80