//------------------------------//
// The Answer 21 
// Story: Life is a Test: A Series of Pony Logic Puzzles 
// by Brony_of_Brody 
//------------------------------//

You can do it with as few as three operatives.

Don't see how?

Well, if you're still reading:

123 - 45 - 67 + 89 = 100.

Brilliantly simple.