It’s really a test of aptitude rather than knowledge per se. It’s highly correlated with IQ. It’s used to help decide the kinds of roles a person may be qualified for on the basis of ability, rather than knowledge. They will train a person in whatever specific skills are needed.

I agree, although I think their main concern at this time is to recruit bodies for following orders than math their way into their careers.

No military strength if no military numbers.

If you're doing the kind of ranging that requires arithmetic you've probably specialised and received training in it.

Regular man with gun ranges by sight.

why would you do math in your head if you had pen and paper?

Uh, why wouldn't they? Calculators check a lot of boxes that militaries care about: cheap, light, compact, durable, easy to manufacture, easy to use. I'd think that they'd be standard issue for any personnel regularly tasked with doing important calculations under logistically difficult scenarios.

Setting that aside... you are aware that modern battle tactics are completely, hopelessly reliant on battery operated devices, right? If a unit gets unintentionally separated from the grid long enough to start running out of power, there will be far greater fish to fry than a few botched arithmatic problems.