Guilty! For the purists:
The ISA temperature at FL300 is (30 x -1.98°) + 15° = -44.4°
The temperature deviation is therefore (-57.5°) - (-44.4°) = -13.1°
The temperature correction is 4% for every 10° deviation from ISA which is:
(-13.1°/10) x 30,000ft x 0.04 = -1572ft
The true altitude is therefore 30,000ft - 1,572ft = 28,428ft,
closest answer (C)
I would always use 2° per thousand feet. Its only an approximate calculation anyway.