\frac{\frac{15!}{(16-r)!}}{\frac{16!}{(18-r)!}}=\frac{3}{4}\\ \frac{15! }{ (16 - r)!} × \frac{(18 - r)! }{ 16!} = \frac{3}{4}\\ \frac{15! }{ (16 - r)!} × \frac{[(18 - r) (18 - r - 1) (18 - r - 2)!]}{(16×15!)} = \frac{3}{4}\\ \frac{1}{(16 - r)!} × \frac{[(18 - r) (17 - r) (16 - r)!]}{16} = \frac{3}{4}\\ (18 - r) (17 - r) = \frac{3}{4} × 16\\