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%