The base is 5 as 5 x 20 = 100 and when solving the area of a triangle you divide by 2 to get 50. Hope this helps.
Work: A = h*b/2
50 = 4b*b/2
50*2=100.
from there you use logical reasoning and can decipher that 20 x 5 = 100.
Checking work:
5 x 4 = 20 making the height (20) 4 time greater than the base (5).