Alejandra has five hair ribbons measuring 8,13,11,9, and 13 inches long what is the median length of the ribbons

I'm afraid it's not 11 (unless you can round the result up, but there is no mention about it).

8 + 13 + 11 + 9 + 13 = 54

54 / 5 = 10,8

Firstly you sum all the numbers and then you divide the result by the amount of the summed up numbers.