The length of the sides of any right triangle are always less than the length of the hypotenuse. The sine and cosine functions find the ratio between a side and the hypotenuse, so it's always less than 1.
The expression to represent this would be (m-18)/k.
We first subtract the 18 beads she used in her mother's bracelet from her beginning number, m. We then divide our result by k, the number of bracelets she made for her friends.