For small angles of climb, climb angle is T-D/W. Three engines give 90,000N thrust. Aircraft Mass is 50,000kg, so weight is 500,000N. Drag is 1/10 of lift, equals 50,000N

 

Climb angle (or gradient, it’s the same thing at small angles) is:

 

(90,000 - 50,000)/500,000 = 0.08radians

 

This is about 4.5deg, or 8.0% (from the answer 4 up in 50 along)