Solution:
The shape from which we will count the lattice points has four vertices. Each vertice is a solution of the ordered pair (3s + 7t, 5s + t) with (s,t) as (0, 0), (0, 1), (1, 0), (1, 1), since these represent the bounds of s and t: 0 < s < 1 and 0 < t < 1. There are 31 lattice points, and thus 31 solutions.
Draw it out and it will make more sense.