Answer: 32 copies of hardcover version.
Step-by-step explanation: Suppose the quantity of hardcover version copies sold is H and the paperback copies is P.
1) A store sells a total of 70 copies:
H + P = 70
2) One hardcover is sold for $15 and one paperback for $11.50. In one month was charged $917:
15H + 11.5P = 917
Solving the system of equations:
H + P = 70 (I)
15H + 11.5P = 917 (II)
Since it is asked for the hardcover version, find H.
Multiply equation (I) by -11.5:
-11.5H - 11.5P = - 805 (III)
Add (II) and (III):
-11.5H - 11.5P = - 805
15H + 11.5P = 917
3.5H = 112
H = 32
In one month, it was sold 32 hardcover version of a book.
