let

x = the number of white sweatshirt

y = the number of yellow sweatshirt

one day a store sold 26 sweatshirts

x + y = 26

the total cost of x white sweatshirts will be 11.95*x

the total cost of y yellow sweatshirt will be 12.50*y

in all, $316.20 worth od sweatshirts were sold that is

11.95*x + 12.50*y = 316.20

by solving the system of equations

x + y = 26

11.95*x + 12.50*y = 316.20

we find

x = 16 white sweatshirts

y = 10 yellow sweatshirts

the store sold 16 white sweatshirt and 10 yellow sweatshirts.