One question, repeated with different numbers: what is this percentage of that number? What is 70% of 40? What is 50% of 14? The percentages are the ones worth knowing - tens, quarters, halves and their neighbours.
This is the percent skill everything else is built on. Finding a percent of a number is what a discount is, what a tip is, and what a test score is; once it is automatic, percent of change is one subtraction more.
Find 10% first by moving the decimal point one place left, then build from it. 30% is three lots of 10%; 5% is half of 10%; 25% is a quarter, which is often faster still.
No. Every question here is chosen so the arithmetic works by hand - the percentage always divides the number cleanly.
The Largest number setting caps the number a percentage is taken of, from 50 up to 500. The percentages themselves stay the friendly ones whatever it is set to.
Sixth grade, and it is used constantly from then on - in ratios, in statistics, and in anything involving money.