Adding large numbers

 Adding large numbers

Adding large numbers just in your head can be difficult. This method shows how to simplify this process by making all the numbers a multiple of 10. Here is an example:

644 + 238

While these numbers are hard to contend with, rounding them up will make them more manageable. So, 644 becomes 650 and 238 becomes 240.

Now, add 650 and 240 together. The total is 890. To find the answer to the original equation, it must be determined how much we added to the numbers to round them up.

650 – 644 = 6 and 240 – 238 = 2

Now, add 6 and 2 together for a total of 8

To find the answer to the original equation, 8 must be subtracted from the 890.

890 – 8 = 882

So the answer to 644 +238 is 882.

cool fact about math

1. “Eleven plus two” is an anagram of “twelve plus one” which is pretty fitting as the answer to both equations is 13.

1. Also, there are 13 letters in both “eleven plus two” and “twelve plus one”.

3. Zero is not represented in Roman numerals.

4. The word “mathematics” only appears in one Shakespearean play, “The Taming of the Shrew”.

5. -40 °C is equal to -40 °F.

6. In France, a pie chart is sometimes referred to as a “camembert”.

7. The symbol for division (i.e.÷) is called an obelus.

8. 2 and 5 are the only prime numbers that end in 2 or 5.

9. A ‘jiffy’ is an actual unit of time. It means 1/100th of a second.

10. If you shuffle a deck of cards properly, it’s more than likely that the exact order of the cards you get has never been seen before in the whole history of the universe

intersting infoabout math

1. The word “hundred” comes from the old Norse term, “hundrath”, which actually means 120 and not 100.

2. In a room of 23 people there’s a 50% chance that two people have the same birthday.

3. Most mathematical symbols weren’t invented until the 16th century. Before that, equations were written in words.

4. “Forty” is the only number that is spelt with letters arranged in alphabetical order.

5. Conversely, “one” is the only number that is spelt with letters arranged in descending order.

6. From 0 to 1000, the only number that has the letter “a” in it is “one thousand”.

7. ‘Four’ is the only number in the English language that is spelt with the same number of letters as the number itself.

8. Every odd number has an “e” in it.

9. The reason Americans call mathematics “math”, is because they argue that “mathematics” functions as a singular noun so ‘math’ should be singular too.

10. Markings on animal bones indicate that humans have been doing maths since around 30,000BC

Multiplying large numbers in your head

calculate a tip

Calculate a Tip

If you need to leave a 15% tip, here is the easy way to do it. Work out 10% (divide the number by 10) – then add that number to half its value and you have your answer:

15% of $25 = (10% of 25) + ((10% of 25) / 2)

$2.50 + $1.25 = $3.75

Easy Multiplication of Numbers

Multiply by 5: Multiply by 10 and divide by 2.
Multiply by 6: Sometimes multiplying by 3 and then 2 is easy.
Multiply by 9: Multiply by 10 and subtract the original number.
Multiply by 12: Multiply by 10 and add twice the original number.
Multiply by 13: Multiply by 3 and add 10 times original number.
Multiply by 14: Multiply by 7 and then multiply by 2
Multiply by 15: Multiply by 10 and add 5 times the original number, as above.
Multiply by 16: You can double four times, if you want to. Or you can multiply by 8 and then by 2.
Multiply by 17: Multiply by 7 and add 10 times original number.
Multiply by 18: Multiply by 20 and subtract twice the original number (which is obvious from the first step).
Multiply by 19: Multiply by 20 and subtract the original number.
Multiply by 24: Multiply by 8 and then multiply by 3.
Multiply by 27: Multiply by 30 and subtract 3 times the original number (which is obvious from the first step).
Multiply by 45: Multiply by 50 and subtract 5 times the original number (which is obvious from the first step).
Multiply by 90: Multiply by 9 (as above) and put a zero on the right.
Multiply by 98: Multiply by 100 and subtract twice the original number.
Multiply by 99: Multiply by 100 and subtract the original number.

Easy multiplication by 11

There are a number of simple techniques that can be used when multiplying by certain numbers. They are very useful in their own right but can be even more useful when combined with other techniques where they can facilitate the solution of more difficult problems.

Multiplying by 11

To multiply any number by 11 do the following:
Working from right to left

1. Write the rightmost digit of the starting number down.
2. Add each pair of digits and write the results down, (carrying digits where necessary right to left).
3. Finally write down the left most digit (adding any final carry if necessary).

It's as simple as that, e.g.

• Multiply 712x11

${\displaystyle {\begin{matrix}&7&&1&&2\\&\swarrow \searrow &+&\swarrow \searrow &+&\swarrow \searrow \\7&&8&&3&&2\end{matrix}}}$

712x11=7832

The reason for working from right to left instead of the more usual left to right is so any carries can be added in. e.g.

• Multiply 8738x11

${\displaystyle {\begin{matrix}&8&&7&&3&&8\\&\swarrow \searrow &+&\swarrow \searrow &+&\swarrow \searrow &+&\swarrow \searrow \\9&\leftarrow _{1}&6&\leftarrow _{1}&1&\leftarrow _{1}&1&&8\end{matrix}}}$

8738x11=96118