The rink has a dimension of 200 ft* 85 ft = 17000

depth = 1.5 inch that becomes
= 0.125 ft
So total volume of the water required = 17000* 0.125 = 2125

Now 1 cubic foot of water= 7.480 gallons
Hence total gallons required are 2125 * 7.480 gallons = 15895 gallons.