1) When the temp. at a give alt is not ISA you need to make corrections to the indicated altitude to get True altitude.
The correction is for every 10*C above/below ISA you need to apply (add/sub) a correction of 4% to the Indicated altitude to get the true altitude.
ISA dev at 12000 ft = actual (9*C) - ISA (-9*C) = 9+9 = 18*C
10*C= 4% correction so 18*C = 7.2 % correction
7.2% of 12000 = 864ft
So since the temp at the given altitude is above ISA add the correction of 864ft to the indicated altitude of 12,000ft.
So the answer is 12,000+864 = 12,864ft.
3) Again similar to Q no.1. Since ISA deviation here is -10*C you will need to subtract 4% of 16000ft from16000ft which is 16000-640 = 15360.
Now since you are flying at FL 160 you will have set the altimeter setting as 1013.25. And your true altitude is based on your QNH which is 1003. So you will have to make an additional correction i.e 1003-1013 = -10 x 27 (1mb = 27ft) = -270
So finally true altitude = 15360-270 = 15090.
4) There are some fixed values given is some books as to the how the value of 1mb changes from S.L upto 40,000 ft.
From what I can recall for every 1 hpa the change in ht. varies like this:
@ S.L= 27ft, @ 2,000ft = 30ft, @20,000ft = 50ft and @ 40,000ft = 100ft
5500m = about 18,000 ft so the closest answer is 50ft.
Sorry about Q no.4.. I'm not really good at explaining stuff as you can see. I'm not able to explain it in words. Perhaps if I could draw I could explain it better. But I will try to put it in words and get back.