1coulomb of charge = 6.25*1018 . how?

I don't understand the question. 
It seems the same as asking "How is 1 mile 5,280 feet ?"

The amount of charge on every electron and every proton is the same.  For human
people, it's a very small amount of charge, so we work with a larger unit, called the
Coulomb.  That's the amount of charge on 6.25 x 10¹⁸ electrons.

In a wire, when that many electrons, carrying that amount of charge, pass by
some point every second, we call the current in the wire 1 Ampere.

Efficiency is an action of doing something. for example when you take a test. good efficiency means you did well but bad effeciency or no effiency means you did bad.

A. to determine if humans can explore mars safely