Mental math at work rarely looks like a worksheet — it's a quick split of a bill, a fast sanity check on a budget line, an on-the-spot estimate in a meeting. Being fast and confident with it is a small, real edge.
Splitting an invoice three ways before the client asks. Rounding a discount to check a quote looks right before you send it. Doing a rough percentage in your head instead of stalling a meeting to find a calculator. None of these need advanced math — they need speed and confidence with the basics, which is exactly what erodes without use.
Find 10% of a number by moving the decimal point one place left — 10% of $84 is $8.40. From there, most percentages you actually need at work are just scaling that number: 20% is double it, 5% is half of it, 15% is 10% plus half of that again. It's faster than it sounds once the 10% step is automatic, which is exactly the kind of single-step recall a 60-second Race round trains.
If a spreadsheet says a total is $19,847 and it should be roughly "41 units at about $480 each," round both to $20,000 and $480 × 40 in your head — close enough to catch a decimal-point error or a doubled line item before it goes out, without needing the exact figure. Estimation is a different skill from precise recall, but it leans on the same comfort with numbers that gets rusty without use.