With of the rink is 9 yards.
Area of a rectangle is computed by multiplying length by its width.
A = lw
A = 900 yd²
l = 100 yd
w = ?
900 yd² = 100yd * w
900 yd² / 100 yd = w
w = 9 yd
to check:
900 yd² = 100 yd * w
900 yd² = 100 yd * 9 yd
900 yd² = 900 yd²