Let, white shirts = x
yellow shirts = y
x+y = 20, x=20-y
11.95x+12.50y=247.25
substitute value of x
11.95(20-y) + 12.50y = 247.25
239-11.95y+12.50y = 247.25
0.55y = 8.25
y = 8.25/0.55 = 15
Now, substitute it 1st equation, x+15 = 20, x = 5
white sweatshirts = 5
yellow sweatshirts = 15