Describe how you could use your height and a yardstick to determine the unknown height of a flagpole.

You measure your height and the height of your shadow the you measure the height of the poles shadow and you then have the equation
y(your height)/s(your shadows height)=x(flagpoles height)/z(flagpoles shadow)
then to find the answer you just multiply the z by y/s

60 ÷ -20 x makes x = -3

No solution : there is no number of boxes that she bought