Estimate with Decimals

Sometimes in everyday life, it is appropriate to use estimation when talking about numbers.

 

 

We may want to calculate an approximate answer to a problem quickly, where we don't need an exact answer.

 

We can use rounding to help us estimate a solution. 

Rounding makes the numbers easier to calculate with, and so can often be done mentally. 

 

Example:

Last month, I spent £187.63 on groceries, £112.37 on transport, and £45.15 on entertainment.

Estimate how much I spent all together?

 

 

I can round £187.63 to £188, £112.37 to £112, and £45.15 to £45.

Now £188 + £112 + 45 = £345

I estimate that I spent £345 last month on the above. 

 

Did you notice that we had decimal numbers in the problem above? We rounded each of them to the nearest whole number.

Let's do the same in the next problem.

 

Example:

A pizza has 8 slices, and I estimate that each person will eat 3.5 slices.

Estimate how many pizzas I will need for a pizza party for 15 people. 

 

 

Digits 0 - 4 round down, and digits 5 - 9 round up, so 3.5 rounds up to 4.

4 x 15 = 60 slices.

60 ÷ 8 = 7.5 pizzas. 

We'd better round that up to 8 pizzas since we can't make half a pizza!

 

Let's do some more estimation practice.