To find the cost of each book, you need to divide the money spent on all of the books by the number of books.
$39 / 10 = $3.90 per book


now, bear in mind, the square root gives us +/- versions, so, which is it? well, we know the angle is in the range of "<span>0 ≤ θ < π/2", that simply means the 1st quadrant, so, we'll use the positive one then
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First you have to have the absolute value isolated so the 2 has to be divided. Then set up the two cases and solve.
You cannot take the square root of a negative number and get a real number answer. You can only get an imaginary root.
square root of 49 = 7 and square root of 64 = 8
square root of 61 is between 7 and 8.
It would be closer to 8.
square root of -49 = 7i and square root of -64 = 8i
square root of -61 would be closer to 8i
![\bf \stackrel{\textit{cost of the pens}}{0.40x+15}\qquad \qquad \qquad \stackrel{\textit{cost of the sportpacks}}{1.50y+30} \\\\\\\ \stackrel{x=500}{[0.40(500)+15]}~~+~~[1.50y+30] \le 3330 \\\\\\ 200+15~~+~~1.5y+30\le 3330\implies 245+1.5y \le 3330 \\\\\\ 1.5y\le 3085\implies y\le \cfrac{3085}{1.5}\implies y\le \cfrac{6170}{3}\implies y\le 2056\frac{2}{3}](https://tex.z-dn.net/?f=%5Cbf%20%5Cstackrel%7B%5Ctextit%7Bcost%20of%20the%20pens%7D%7D%7B0.40x%2B15%7D%5Cqquad%20%5Cqquad%20%5Cqquad%20%5Cstackrel%7B%5Ctextit%7Bcost%20of%20the%20sportpacks%7D%7D%7B1.50y%2B30%7D%0A%5C%5C%5C%5C%5C%5C%5C%0A%5Cstackrel%7Bx%3D500%7D%7B%5B0.40%28500%29%2B15%5D%7D~~%2B~~%5B1.50y%2B30%5D%20%5Cle%203330%0A%5C%5C%5C%5C%5C%5C%0A200%2B15~~%2B~~1.5y%2B30%5Cle%203330%5Cimplies%20245%2B1.5y%20%5Cle%203330%0A%5C%5C%5C%5C%5C%5C%0A1.5y%5Cle%203085%5Cimplies%20y%5Cle%20%5Ccfrac%7B3085%7D%7B1.5%7D%5Cimplies%20y%5Cle%20%5Ccfrac%7B6170%7D%7B3%7D%5Cimplies%20y%5Cle%202056%5Cfrac%7B2%7D%7B3%7D)
or namely 2056 sportpacks if we toss away the fraction.