The question you opened
The 3 percent foreign fee
Your card adds a 3 percent foreign fee to every charge abroad.
What is the fee on a 250 dollar hotel bill, in dollars?
$7.50
One percent of 250 is 2.50, and three copies of it come to 7.50. Finding one percent first keeps every number small enough to hold.
The fee is 3 percent of 250 dollars. One percent of any number is that number with the decimal point moved two places left, so one percent of 250 is 2.50. Three of those is 7.50, and the hotel bill costs 257.50 in total. The method generalises to anything under about 20 percent. Get one percent, then add copies. Seven percent of 250 is seven copies of 2.50, which is 17.50.
Four percent of 1,800 is four copies of 18, which is 72. The multiplication stays in single digits, which is where mental arithmetic is reliable. Above 20 percent it is faster to build from 10 percent and halves. Ten percent of 250 is 25. Five percent is half of that, 12.50. Fifteen percent is 37.50, and 35 percent is 25 plus 25 plus 25 plus 12.50, so 87.50.
Every common percent is a sum of tenths, halves of tenths, and hundredths, and none of the pieces needs paper. Foreign fees are worth the practice because they hide. Three percent looks like rounding until it is applied to a trip. Two weeks abroad at 200 dollars a day is 2,800 dollars of spending, and 3 percent of that is 84 dollars, which is a night in a hotel. Card providers that advertise no foreign fee are selling exactly this number back to you.
One more piece belongs in the same habit. Some banks apply the fee to an exchange rate they set themselves, so the true cost is the fee plus the spread. Comparing cards means comparing both, and the only way to see the spread is to check the rate you were charged against the rate on the day.
Technique: Find one percent, then copy it