Correct Answer - Option 3 : 18
Given:
Average of first four numbers = 30
Average of last four numbers = 28
Fifth number = 10
Formula used:
Average = (sum of all observation)/Number of observation
Calculation:
Sum of first 4 numbers(I, II, III and IV) = 30 × 4
⇒ 120
Sum of last 4 numbers (II, III, IV and V) = 28 × 4
⇒ (II + III + IV + 10) = 112
⇒ (II + III + IV) = 102
⇒ (I + II + III + IV) = 120
⇒ I + 102 = 120
⇒ I = 120 - 102
⇒ I = 18
∴ First number is 18.