How_To_Be_Good_At_Math

NUMBERS • ESTIMATING 25

Estimating a total

Sometimes we estimate because it would take too

long to count or calculate the exact answer.

Checking a calculation

Sometimes, we work out what we expect an answer

to be by simplifying, or rounding, the numbers.

There are 12 tulips

in the bottom right

square. So, the total

number is approximately

12 × 9, which is 108.

Our two estimates have come up with

answers of 99 and 108. In fact, there are

105 tulips, so both estimates were pretty close!

When we do the actual

calculation, the answer we

get is very close to our estimate.

So, we can be confident that our

addition is correct.

Another way to

estimate the total

is to divide the bed

into rough squares.

If we count the flowers

in one square, we can

estimate the number

in the whole bed.

The first number is slightly

less than 3000, and the

second is slightly more than

4000. We can quickly add 3000
      to 4000, to get 7000.

Let’s look at this

bed of tulips. We

want to know roughly

how many there are,

without having to

count them, one

by one.

Let’s add together 2847 and

4102. We make an estimate
      first so that if our answer is very
      different, we know that we might
      have made a mistake.

The tulips aren’t

in exact rows,

but we can count 11

flowers in the front

row. There are nine

rows, so we can

say there are about

11 × 9 flowers, which

is 99.

2847 + 4102 =? 3000 + 4000 = 7000 2847 + 4102 = 6949

There are nine

horizontal rows

The flower bed is

divided roughly

into nine squares

There are 11 flowers

in the front row

We estimate that the answer

will be approximately 7000

There are 12 flowers in

the bottom right square

024_025_Estimating_Numbers.indd 25 29/02/2016 14:08