The worst part of going out to dinner with a group of friends is figuring out who owes what at the end of the night. All too often, once everyone's chipped in, there's either not enough money on the table, or way too much.When trying to plan for retirement, or figure out whether to make an investment, one helpful thing to know is how long it will take to double your initial capital.And when you're considering taking a new job with an annual salary, it might be useful to know how that salary compares with what you'd earn at a job with an hourly pay rate.Some handy math tricks can help you figure out each of these situations quickly ' and without having to resort to pulling out the calculator on your phone. We've assembled seven such tricks and talk a little about how they all work.Eric Platt and Walt Hickey contributed to earlier versions of this article.WATCH:We Did The Math: Which iPhone 6 Should You Buy'Converting your salary to an hourly figureThe Trick: Take your salary, drop the last three zeros, and then divide by two.Example: So if you earn $40,000, you're left with $20 an hour.Maybe you're a salaried employee trying to figure out whether you should take that job that pays by the hour. This trick would certainly help.This comes from making a couple of assumptions about your working hours and some quick observations about time. There are 52 weeks in a year, and so if you're working a 40 hour week for 50 of those weeks, you'll be working 40 x 50 = 2,000 hours in a year. Take your annual salary and divide by those 2,000 hours (equivalently, drop the last three zeros and divide by two), and voila, you get your hourly rate.Figuring out sales tax and tipsThe Trick: Assuming an approximately 9% sales tax, as in New York, if you take your pre-tax part of the bill, divide by four, and add this amount to the pre-tax part, you'll be leaving about a 16% tip. If you're feeling more generous, divide by three and add that amount instead, leaving about a 24% tip.Example: Say your part of the pre-tax bill comes to $18. If you want to leave a 16% tip, divide $18 by four to get $4.50. Add that to your $18 pre-tax cost, and leave a total of $22.50. If you want to leave a larger tip, divide $18 by three to get $6, and combine that with the pre-tax cost to leave a total of $24.Here's why this works: Dividing the pre-tax bill by four is the same as taking 25% of that amount. That's enough money to cover the 9% sales tax, with 16% leftover for the tip, because 25 - 9 = 16. Similarly, dividing by three gives about 33% of the pre-tax bill, covering the 9% sales tax and a 24% tip, because 33 - 9 = 24.Business Insider's Sarah Schmalbruch alsorecently wrote about a few other nice tricks to calculate tips, like moving the decimal point one place to the left and multiplying by two to find 20% of the bill.So, if the bill is $18, the moving the decimal points gets you $1.80. And multiplying it by two gets you $3.60.How long it'll take to double your investmentThe Rule of 72: Need an easy way to determine how long it will take to double your investment' Simply divide the number 72 by your projected growth rate.Example: So, if you're getting a return of 9% per year, it will take about eight years for your investment to double in size, because 72/9 = 8.The rule of 72 comes from manipulating the basic mathematical formula for calculating compound interest. The exact number you need to divide your growth rate by is actually closer to 69, but 70 or 72 are frequently used for quick mental estimates, because most common single-digit rates of return will divide evenly into one of these.See the rest of the story at Business Insider
Click here to read full news..
