"At least half" means that he gets 8 or more correct.
Probability of getting 8 correct
C(15,8)(1/2)^8(1/2)^7 = 6435/32768
Probability of getting 9 correct
C(15,9)(1/2)^9(1/2)^6 = 5005/32768
Probability of getting 10 correct
C(15,10)(1/2)^10(1/2)^5 = 3003/32768
Probability of getting 11 correct
C(15,11)(1/2)^11(1/2)^4 = 1365/32768
Probability of getting 12 correct
C(15,12)(1/2)^12(1/2)^3 = 455/32768
Probability of getting 13 correct
C(15,13)(1/2)^13(1/2)^2 = 105/32768
Probability of getting 14 correct
C(15,14)(1/2)^14(1/2)^1 = 15/32768
Probability of getting 15 correct =
C(15,15)(1/2)^15(1/2)^0 = 1/32768
So....the total probability is = [6425 + 5005 + 3003 + 1365 + 455 + 105 + 15 + 1 ] / 32768 =
16374/32768 = 8187/16384