Mental math is doing arithmetic in your head, without a calculator. It's what lets you split a bill three ways, tell whether 30% off is actually a good deal, or catch a wrong total before it leaves the till. Like any skill, it gets rusty without use — the good news is that a few minutes a day is enough to bring it back.
Two things move the needle: short, frequent sessions rather than one long one, and a level where you get most answers right. Too easy and you're not building anything; too hard and you're guessing. Aim for a level where you're right about 80–90% of the time, and let it get harder as that number climbs.
A timer helps too. Working under a small amount of pressure is closer to real life — checking a bill at the table, not at a desk with a pad of paper — and it's why games built around a clock train differently to untimed drills.
One Race round on a level where you get 80–90% right. Ninety seconds, done.
Check Stats. If one operation lags behind, spend a round or two on that difficulty on purpose.
Move up a level. Rookie → Amateur → Pro → Elite. Elite unlocks after 10 Pro wins.