Adding Numbers Quickly
So what does it mean to estimate in math? Let's start off with a simple example. Suppose we're doing the grocery shopping and want to keep a running total of how much we're going to spend; maybe we have $100 to spend and want to avoid going over that. If we buy items that we estimate add up to no more than $80, we can be off by $10 and still have enough left over for sales tax!
So let's say we've already selected items that cost $1.23, $5.98, $1.99, $19.99, $3.25, and 99c. How much have we spent so far?
You could write everything down and add it up, but an easier way is to simply round everything to the nearest dollar and add those. Rounded off, we have $1, $6, $2, $20, $3, and $1; it's easy enough to mentally add those up and find out that we've spent about $33 so far. (The actual total is $33.43, so our estimate is off by 43 cents, or about 1.2%) Every time we add another item to our shipping card, we just round off that price and add it to our running total.
In fact, because prices tend to end in .97 or .99 more often than not, estimating in this way will often cause you to slightly overestimate how much you've spent, which is always good for your wallet!
Ready to practice? First estimate the sum of these numbers, then add them up to find the actual result: 42, 51, 39, 88.