f(x) Damn spell checker won’ let me do it right with the non-capital f. You could do the first one as .1(75)/2 - .(01)75=3. Or 3.75 - .75 = 3 I do it in my head by using 10s and 1s.

I don’t know how people live with out doing mental calculations. If nothing else, you could use a slide rule (no electricity).