How to calculate true altitude

[Nooob]

Can I please know why the answer is 19,340ft?

You are cruising at FL 200, OAT -40° C, sea level pressure 1033 hPa. Calculate the true altitude?

A) 21740 ft. B) 18260 ft.

C) 19340 ft.

D) 20660 ft.

Thank you!

[eckhard]

ISA dev is -15

Alt error is approx 4ft per degree dev per 1000ft

So error is 4x15 = 60ft per 1000

60 x 20 = 1200

So true Flight Level is 188

QNH = 1033, so approx 20 x 30 = 600 ft correction

18800 + 600 = 19400