Let
x = number of school bags
y = number of sweaters
z = number of color pencils.
They want to buy 200 items translates to
With a total $2800, a price of $20 on school bags, $25 on sweaters and $5 on color pencils translates to
They must order as many color pencils as school bags and sweaters combined, translates to
Therefore, we have the following system
Substitute the z of the third equation to the first and second equation
Solve the system of fourth and fifth equation using elimination method.
Substitute the value of x to the fourth equation and solve for y
Finally, substitute the value of x and y to the third equation to solve for z
Since y is the number of sweater, the number of sweaters that they have ordered is 60.