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Cheat Sheet

Everyday Math For Dummies

Everyday math comes in handy when you’re dealing with finances like credit cards and mortgages, and even helps when you’re trying to figure out how much to leave for a tip. Knowing some basic math formulas, the Pythagoras’ theorem, and a simpler way to add are key to everyday math.

Grouping Numbers for Easy Addition

If you’re looking for a quicker way to do addition, use this chart to group numbers. Put your list of numbers on paper and then follow this example:

Numbers Pair Add Pair Again Add Total
17 (a)
151 (d)
23 (a) (a) 23 + 17 = 40 (e)
24 (b) (b) 24 + 66 = 90 (f) (e) 190 + 90 = 280
205 (c) (c) 205 + 75 = 280 (f) (f) 280 + 40 = 320 280 + 320 = 600
39 (d) (d) 39 + 151 = 190 (e)
75 (c)
66 (b)

Using Basic Math for Tipping

These basic tipping rules apply to meals in the $20 to $100 price range, which covers almost 90 percent of restaurant meals for two in the U.S. Apply these simple math rules to your check total:

  • Basic Rule (total between $30 and $100):

  • Multiply the first digit by 2. That’s the tip.

  • Example: $43.75. The first digit is 4. Leave 4 ¥ 2 = $8.

  • Lower-cost rule (total between $6 and $30):

  • Divide by 5 and round off. That’s the tip.

  • Example: $26.90. Five goes into this about 5 times. Leave $5.

  • Party of four (total between $40 and $100):

  • Round to the nearest 10. Multiply the first digit by 3. Everyone kicks in that amount.

  • Example: $52.50. Rounds to $50. Everyone pays $15.

If you’re not up for doing the math, use this handy tip chart. The tip range is 15 to 18 percent but if you’re feeling generous, feel free to tip more.

Total on Check Tip Total on Check Tip
$10 and below Be nice. $84 to $95 $16.00
$11 to $16 $2.50 $96 to $110 $18.00
$17 to $23 $3.00 $110 to $130 $20
$24 to $30 $4.50 $131 to $160 $25
$31 to $37 $5.00 $161 to $200 $30
$38 to $44 $6.00 $201 to $250 $40
$45 to $52 $7.50 $251 to $300 $50
$53 to $60 $9.00 $301 to $400 $60
$61 to $70 $10.00 $401 to $500 $80
$71 to $83 $12.00

Getting to Know Pythagoras’ Theorem

An earlier version of Pythagoras’ theory existed in ancient civilizations, but only for particular triangles. The Pythagoras’ theorem expanded this earlier version, try the proof for yourself:

image0.jpg

Using Everyday Math in Everyday Life

Whether you’re dealing with your everyday finances, or getting together for a neighborhood card game, you need to know basic math to help you simplify and stay on top of your money matters.

Playing poker

Here are a few crucial rules that you can memorize easily. Spend your time in a real game watching people tap their feet or blink a lot.

  • In a four-person game, you need to have a hand with at least a pair of tens to have a 50 percent chance of winning.

  • If you really like to have things nailed down, you should have at least two pair, kings high to have a 90 percent chance of winning.

Using credit cards

Warning: If you make a purchase on a credit card with an interest rate near 20 percent and then make payments at twice the monthly minimum, the purchase will cost 50 percent more than the price in the store.

Dealing with mortgages

Here’s a simple mortgage table for a $100,000 loan. To calculate a different loan amount, multiply the payment by the new amount and divide by 100,000. For fractional interest rates, take the fraction of the step up to the next whole-number rate.

Interest Rate Monthly Payment, 30 years Monthly Payment, 15 years
4% $477.42 $739.69
5% $536.82 $790.79
6% $599.55 $843.86
7% $665.30 $898.83
8% $733.76 $955.65

Basic Math Formulas

If you’re looking to find the area or volumes of basic shapes like rectangles, triangles, or circles, keep this diagram handy for the simple math formulas:

image0.jpg
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