I teach math and I'm always looking for better quick calculation methods to share with my students. For example, when multiplying by 5, I tell them to just multiply by 10 and then divide by 2. That's way faster than doing the actual multiplication.
What other time-saving math methods do you use regularly? I'm particularly interested in mental math tricks that work well for grocery shopping, cooking, or just general everyday calculation shortcuts.
I love the multiply by 5 trick! Another one I use all the time for grocery shopping is calculating 10% quickly by moving the decimal point one place to the left. Then for 20%, just double that. So if something is $4.50, 10% is $0.45, 20% is $0.90. Makes tip calculation shortcuts so much easier too.
For cooking, I use a lot of quick calculation methods with fractions. Like if I need 3/4 cup but only have 1/4 cup measure, that's easy - three of them. But the real time-saving math method I use is for temperature conversions. Instead of the exact formula, I use 2x + 30 for Celsius to Fahrenheit. It's not perfect but close enough for cooking.
When traveling, I use the rule of 72 for quick calculation methods with time zones. Every 15 degrees longitude is 1 hour time difference. So if I'm going 45 degrees east, that's 3 hours ahead. Not exact but close enough for planning calls back home.
For shopping, I have a mental quick calculation method for sales tax. Where I live it's 8.25%, so I just take 10% and subtract a little. Actually easier: 8.25% is about 1/12, so divide by 12. $60 item? Tax is about $5. These everyday calculation shortcuts save me so much mental energy.
I use fraction calculation shortcuts all the time. For quick calculation methods with thirds, I remember that 1/3 is about 33%, 2/3 is about 67%. When doubling recipes, if it calls for 2/3 cup, doubled is 1 and 1/3 cups. I just think two of the 2/3 cups" which is 4/3, same as 1 and 1/3.
My favorite quick calculation method is for percentages. 15% tip? Take 10%, half it for 5%, add together. So $24 meal: 10% is $2.40, 5% is $1.20, total $3.60. Round to $4 if service was good. These real-life problem solving math tricks make dining out less stressful.