How to Make Estimates (Mental Math)

If you’re at the store buying three bananas for $\text{\$}1.07$ per banana and one gallon of milk for $\text{\$}1.92$, it’s easier to make an estimate than to find the exact sum. Using the rules above, the calculation is as follows:

You round the price of the banana down from $\text{\$}1.07$ to $\text{\$}1.00$. That makes the estimated price of the bananas $\text{\$}1.00\cdot 3=\text{\$}3.00$. The price of the milk, $\text{\$}1.92$, can be rounded up to $\text{\$}2.00$. The estimated price of your items is

The real price of the items is

which isn’t far off from $$5.00$. In this example, you might have noticed that the estimate is lower than the actual price. This is unfortunate, because you might have tricked yourself into believing you had enough money. For that reason, you should always make your estimate at the store in such a way that it’s higher than the actual price. That way, you are certain to have enough money. You could do like this, for instance:

You round the price of the banana from $\text{\$}1.07$ to $\text{\$}1.00$. Then, the estimated price of the bananas is $\text{\$}1.00\cdot 3=\text{\$}3.00$. You can estimate the price of the milk, which is $\text{\$}1.92$, to be $\text{\$}2.50$. Your estimate of the price is then

If you have $$5.50$, you know that you have enough to pay for the items.