Connie, Ken and Dan took a Mathematics test. Connie scored 3 marks above their average score. Ken scored 14 marks fewer than Connie. Find the difference between Dan’s marks and Ken’s marks.

if the average is x, then

c+k+d = 3x
c = x+3
k = c-14 = x-11
that means that
(x+3) + (x-11) + d = 3x
d = x+8
thus, d-k = (x+8)-(x-11) = 19