Let x be the cost of 1 pair of pants, and y be the cost of one shirt, then since five shirts and three pairs of pants cost $113. 25 we can set the following equation:

also, a shirt costs $3.75 less than a pair of pants, then:

Substituting the second equation in the first one we get:

Solving for x we get:

Now, substituting x=16.5 in the second equation we get:

Answer:
A shirt costs $12.75, and a pair of pants cost $16.5.