Sum of the digits of the number=5+4+7+a=16+a. Then, all the possible values of a for which 16 + a is divisible by 3 are 2, 5, 8, 11,.... But the number is 4-digit only. Then, only possible values of a are 2, 5, 8. Now, the sum of possible values of a for which the 4-digit number 547a is divisible is,=2+5+8=15