If x represent the number of nickels in the box and y represent the number of dimes in the box (note that x>0 and y>0), then the total value of x nickels is $5x and total value of y dimes is $10y. Adding these amounts of money you get that in money box was $(5x+10y).
Sally knows that she has less than $20 in the box, then 5x+10y<2000 ($20=2000 cents).
Since this inequality is strict, then you should draw the dashed line 5x+10y=2000. The line divides coordinate plane into two parts.
Pick point (0,0) and consider to which part this point belongs. Since 5·0+10·0<2000, you can conclude that (0,0) is a solution of given inequality and belongs to needed part. Hence you should take the shaded part below the line for positive values of x and y.
