The average of 20 numbers is 12. The average of the first 12 numbers is 11 and that of the next 7 numbers is 10. The last number is :

28 38 48 14

38

Last number = (Sum of 20 numbers) - (Sum of first 12 + Sum of next 7)                     = (240) - [(12 × 11) + (7 × 10) ]                     = (240) - (132 + 70) = 38