Very good read this. Some simple tips to calculate statistics faster.

Notes:

• To multiple tens / hundreds. First times the digits. Then add on all the zeros.
• If the VAT is 20% you can find it by dividing by 1.2 - that's the same as 5/6... so you can get the VAT by dividing by 6 and taking the remainder.
• 16% of 25 is the same as 25% of 16 (4). Flipping the percentage and number can make maths easier
• Zequels is a rough equals. It comes from rounding numbers to the nearest 0 value and adding/dividing/subtracting/multiplying. It's useful to get a ballpark figure. If you've an excel number that's no where near the zequels there's probably been a forumla error. You can be more or less discerning / specific with zequels as needs arise. Eg if there's 2 .5 you can round one up and one down.
• Square root. split numbers into groups of 2 digits. Square root the first pair of digits (or single digit). Convert the remaining sets of digits to one 0. You're in the right ballpark. Eg 100615. 10 06 15. squareroot 10 = 3.something. add the 2 sets of 2 digits. 300ish
• Conversions. Litres to pints is roughly double. km to miles is roughly half. cm to in is roughly half.
• Don't settle for your first zequels. Refine with discernment. Eg how many words in a book. Count lines in a page, count pages, multiply. You can improve by taking a sampling of lines on pages and using the average first before the multiplication though.