Learning to estimate develops a valuable life skill such as estimating the cost of items or calculating distances. Sometimes, however, the idea of estimating throws people off because estimating doesn't require you to get the exact answer -- which is a rare concept in math.
When to Use Estimates
Most things in life don't require exact numbers, but generally, you want to know if you have enough money to pay for a trip, buy food or pay rent. Estimating gives you a bit of a cushion for unexpected expenses, as a few pennies here and there won't affect you very much. Suppose you go to the store and buy five items that cost $1.85 each. You know this should cost just slightly more than $9, but the cashier asks for $15.95. By estimating your total, you quickly determine that the cashier made a mistake.
How to Estimate
The type of estimation you use depends on the circumstances. In math class, the teacher generally tells you to estimate to the nearest dollar, nearest hundred or nearest thousand. In real life, what you estimate is based on your needs. If you only have $10 to spend, and one item costs $4.95 and the other costs $5.95, round up to the nearest dollar. You will see that you're short a dollar. Let's say you need to estimate the number of houses in a city. If you see 95,400 houses in one of the counties in that city and 18,400 in another county, you round 95,400 to 100,000 and 18,400 to 20,000 homes. This gives you about 120,000 homes in those two counties in your hypothetical city.
When you go to the grocery store, estimate your bill. Round everything up to the nearest dollar and see how close you get the total at the register. Figure out if you get a better value by buying one pound of coffee at $9.95 a pound or one-half pound for $6.95. Round $6.95 up to $7 and then multiply by two. If you buy the coffee at $9.95 a pound, you will get 16 ounces for just under $10. If you buy two 8-ounce, half-pound bags at $6.95 per half pound, then a pound of coffee will cost you nearly $14.
Estimating Instead of Counting
Think of how many times you needed to know approximately how many items a container held. Look at a jar of jelly beans. You can't count every jelly bean, but you can make a guess. One way is to count all the jellybeans along the bottom of the jar. For best results, lift the jar. If you can't lift the jar, then estimate. Don't worry about counting every bean, just get a general number. After you've counted the approximate number of jellybeans along the bottom of the jar, count how high the jellybeans go up to in the jar. If the jar is 10 inches high from its neck to the bottom and the jellybeans start one inch below the neck, you see that there are nine inches of jellybeans in the jar. Multiply the number of jellybeans you counted on the bottom of the jar by nine inches in height. Multiplying these two numbers together will give you a close estimate as to how many jellybeans there are in the jar.
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