This question should not appear in UK exams.
But in the German LBA exams this question is NOT multiple choice, the calculated answer must be written 3500.
The formula required to answer this question is:
Total thrust = Mass thrust + Pressure thrust
or T = m(Vj - Vv) + A(pj - p0)
where m = mass air flow, Vj = exhaust velocity, Vv = TAS,
A = exhaust cross section area, pj = exhaust static pressure and
p0 = static ambient pressure.
In this instance, the pressures are given in hPa, and need to be converted to
n/sq m.
Therefore, T = 50(150 - 90) + 0.1(105000 - 100000), which gives the answer of
3500N