9x -3y= 24. -7 +3y =-20

The surface area for this case is given by:
 A = 2 * pi * r ^ 2 + 2 * pi * r * h
 Where,
 r: radius of the coins
 h: height of the coin stack.
 Substituting values we have:
 A = 2 * 3.14 * (1/2) ^ 2 + 2 * 3.14 * (1/2) * (100 * (1/8))
 A = 40.82 in ^ 2
 Note: the correct thickness of the coins should be 1/8 in.
 
 Jose will need 40.82 in ^ 2 to cover the stack of coins