This question assumes that lift = weight and thus the followinf formula can be used:
Gradient = (thrust/weight - drag/lift) x 100%
Mass of aircraft = 50,000kg = 50,000 x 10 = 500,000N
Total Thrust = thrust per engine x No. of operating engines.
In this question it states that although it is a twin engine jet
aeroplane and that all engines are operative.
Total thrust = 60,000 x 2 = 120,000N
Inserting the information into the formula gives:
Gradient = (120,000/500,000 - 1/12) x 100
= (0.24 - 0.083) x 100
= 15.7%