Grenades are thrown weapons. Thrown weapon is (Strength + Dexterity) divided by 2, the result being rounded down. So if you have 3 strength and 2 dexterity, you have 2 thrown weapon skill.
Explosives skill is made up of two skills, Computer Operation and Perception. However, there is no equation an endskill, just for a percentage of setting up the bomb, which is (CompOps x 20) + (Perception x 5) = End percentage. So a CompOps skill of 2 and a Perception skill of 4 will give you a percentage of 60% of successfully setting up the bomb. 3 CompOps and 3 Perception however gives you a 75% however.