Answer:
cos X = 3/5
Explanation:
Cosine is a ratio. The side (leg) adjacent to Angle X is the side (leg) next to the angle. The hypotenuse is the longest side.
Cosine is the ratio of the Adjacent side /Hypotenuse.
In this image,
cos X = adj/hyp
cos X = 15/25
cos X = 3/5
cos X = 3/5
(Like the weird wording in the question:
3 fifths)