(18)/100 = 18% ; 5/(50)=10% ; why when the two numbers added together (13)/50 = 26%? I thought the answer should be anywhere between 10% to 18%?

Your question has a mistake.

18/100 = 9/50

Therefore, if you add 9/50 + 5/50 it gives 14/50(not 13/50). This is the same as 28%.

This is the sum of the number and the sum of %. Not average.

Average is (9/50 + 5/50 )/2 = 7/50 = 14%.