Density height calculation question
I have an assessment for the Qantas pilot academy. They have sent me a practice test with the following question:
"The elevation of Echo is 3480ft AMSL. The QNH is 1021 hPa and the temperature is 24C. What is the density height of Echo?"
A. 1920 ft
B. 5105 ft
C. 3820 ft
D. 5560 ft
I calculate this as follows:
PH = 3480 + (1013-1021) x 30))
= 3480 - 240 = 3240 ft PH
ISA Temp @ 3240 PH = 15 + (3 x -2) = 9C
ISA Deviation = OAT - ISA
= 24 - 9 = +15C
Density Height = PH + (ISA Deviation x 120)
= 3240 + (15 x 120)
= 5040 ft DH
Can someone please explain what I'm doing wrong?