Hi there!!
First, Let plain shirts be x
Let fancy shirts be y
x + y = 7
15x + 28y = 131
Multiply first equation by 28 to eliminate y:
28x + 28y = 196
15x + 28y = 131
Subtracting the two equations:
13x = 65
x = 5
Substituting x to get y:
x + y = 7
5 + y = 7
y = 2
Answer: 5 plain shirts and 2 fancy shirts
Hope this helps u!!
Have an amazing rest of ur day:)))