How many functions f:{1,2,3,4}→{1,2,3,4,5,6}f: \{1, 2, 3, 4\} \to \{1, 2, 3, 4, 5, 6\}f:{1,2,3,4}→{1,2,3,4,5,6} satisfy f(1)<f(2)<f(3)<f(4)f(1) < f(2) < f(3) < f(4)f(1)<f(2)<f(3)<f(4)?
15
20
35
56