This is from the quant practice problems. Granted, I am super dumb at math and I don’t always understand the “leaps” taken in solving a problem, but it looks to me as if the solution is for a different problem. (The entire rest of the solution goes on these quantities.) Or can you actually rearrange quantity A and quantity B this way?
You can - this is called “manipulating QC quantities”. So it’s just rearrangement:
f(40) - f(60) vs f(-60) - f(-40)
f(40) - f(60) + f(-40) vs f(-60)
and so on