About the EXP from the pyramid...

how much EXP do we get in rank A, B, C in each mode? (Easy, normal and etc...)

It's a mystery.
Though, I have some stats just in case it helps.

Mode	Kill	Cool	Miss	Grade	EXP
Easy	2951	190	67	S	68302
Normal	3173	146	63	S	73806
Normal	3440	195	32	S	74830
Normal	3459	199	71	S	74908
Normal	2950	164	38	S	73540
Hell	39	0	0	D	78

So, it seems like...
Each KILL is worth 2 EXP.
Each COOL is worth 10 EXP.
Each MISS is worth 0 EXP.
Each S rank in EASY mode is worth 60500 EXP.
Each S rank in MEDIUM mode is worth 66000 EXP.
Each S rank in HARD mode is worth 71500 EXP.
Each S rank in HELL mode is worth 77000 EXP.

hard mode s ranks are somewhere around 70K

You forgot something.

If you get an S Rank in Hell Mode you'd get about 86k-87k EXP.

You forgot something.

If you get an S Rank in Hell Mode you'd get about 86k-87k EXP.

It's 77k + kill*2 + cool*10.

How to find the volume of free space inside a pyramid of 4 spheres.? There are 4 spheres, 3 making up the base with the other stacked on top in the middle. Each sphere has a radius of 1.85u. In the middle of all the spheres, there is an empty space. How can I know where this space ends (as it meets with empty space outside the pyramid), and how can I calculate its' volume?