If you draw the little triangle showing weight, thrust and drag in the
climb then you find that angle of climb is Thrust
minus Drag divided by Weight x Cos Climb angle.
At small climb angles, the sort of angles we get with transport
aircraft, the Cosine of the climb angle is very nearly one, so we can say that
the climb angle is (T-D)/W
The question gives you two engines at 28,000N each, so T is 56,000N
Aeroplane mass is 50,000kg, so weight is 500,000N (Mass x g)
Assume for small climb angles that L = W. Lift/drag ratio is 12, so drag
is 500,000/12 = 41,667N
Put these figures into the climb equation:
Angle of climb = (56,000 - 41,667)/500,000 = 0.02866
Now at small angles this is the climb angle in radians and therefore
approximately both Sin and Tan of the climb angle. Tan of the climb angle is
the actual gradient. To get a percentage gradient you multiply by 100, and the
answer is 2.866, or 2.9.