### Simple Math Advice on Remembering Birthdays and Making Shopping Lists

You know those people who need to recollect the year of their parents', siblings' and even kids' birth, to tell how old these relatives are?
“She was born in April 1998, so she must be 11 years old.”
Well, they may be on to something...

Take me as a bad example. Whenever I hear that one of my relatives' kids has a birthday, I call the parents to ask for the child's age and rush to buy a present for him. When we visit relatives for the holidays, I create long lists of names and terrorize my husband until he calls to find out the exact ages of all his nephews. A year passes and then we call again.
"She is already 13! We can't believe she is growing up so fast."
We should have known. She was 12 the year before...

My new best strategy and my advice: the first time you buy a present for your friend or relative, record the date and year of their birth in your phone book, contact list, or calendar. Next year, to recall their age, all you need to do, is simple subtraction. They will be really surprised you knew! Suggest this to your aunt as well – she may stop embarrassing your 10 year old by asking if he has a girlfriend yet.

Shopping lists.
We love them. We hate them. We live by them.
Except that sometimes we forget to write something obvious like milk or bread, and then as soon as we are back from one shopping trip, we know another is imminent. And as much as we love some of these stores, who wants to go food shopping more than once a week?

Try a new approach - instead of marking what you need to buy (and risking missing something), create a master list of all the things you ever buy, xerox it or print one copy each week. You can start with this Master Shopping List. Before heading to the store, cross out the things you see in your refrigerator. You will not forget milk or bread anymore and believe it or not, you will be playing with the Set Theory.

The products on your list that you need to buy will be a Complement Set of all the products you crossed out because you have them. Every time you discover a new product that you like – expand your original set. If you prefer to keep separate lists for different stores, such as wholesale and local supermarkets, fair enough. There is probably some overlap between those lists. To avoid buying twice, as soon as you are back from one store, cross out the duplicates from your second store list. These duplicates are intersection of your wholesale shopping set and your local supermarket set.

Set Theory applied to shopping lists

Once your universal list is in the computer, it is easy to arrange products to appear in the order you come upon them as you navigate the store. No more random dashes against store “traffic” with a full cart and two kids in tow. And who knows, you may start enjoying it so much that you would not mind food shopping twice a week. :)

Would you like to read more from The Math Mom? Here are a few stories for Valentines Day: The Math of Dating - Mark Yourself Excelled, The Math of Ex.