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.
Phryss isn't an admin. But both are correct. The rules will almost certainly be modified repeatedly well before combat is released at all, so there is no real use in asking about this already.