multiplying and dividing by 10, 100 and 1000?

Question

Im trying to help my daughter with her math but cant remember how to do this without using a calculater. I want to teach her how to solve it without using a calculater because she doesnt get to use one in school. What is the formula for this? example: 8.7 x 100, and 7.8 divided by 10.

Answer 1

When multiplying by 10, 100, 1000, etc., count the number of zeroes and move the decimal to the right that many places.

8.7 times 100 move the decimal right 2 places since there are 2 zeroes . To do this you will need to add another zero and get 870.

To divide, just move the decimal to the left the number of zeroes you have. So 7.8 divided by 10 is .78. Again if you need to you can annex zeroes.

Answer 2

for every zero on the end of a number by which you are multiplying, move the decimal one place to the right, adding zeros if you must.

8.7x100, for example: 100 has two zeros on the end, so I move the decimal place to the right twice. 8.7x100 = 870.

If you are dividing, you move the decimal to the left.
7.8/10 = 0.78
7.8/1000 = 0.0078

I would call this more of a math question than a medicine question.

Answer 3

Yeah, it basically is just jumping around the decimal point. Show her where the decimal is on whole numbers though, so she doesn't get confused (IE - 8. )

From there, it is just a matter of jumping the decimal point based on the problem. Multiplying by 10, 100, 1000, etc. means move the decimal to the right (since multiplying makes the number bigger). Dividing by 10, 100, 1000, etc. means move the decimal to the left (since division means dividing into smaller portions). The amount of digits to move is equal to the number of zeroes.
Multiplying (move decimal to the right) by 10 (one zero)
Multiplying (move decimal to the right) by 100 (two zeroes)
Dividing (move decimal to the left) by 1000 (three zeroes)
And so on...

Repetition helps to retain this too. Best of luck, and good to see a parent taking interest in her child's studies.

Answer 4

one way to do this is to move the decimal point a certain number of places to the right or left. since the decimal point is a tenth of one unit moving it to the right or left one place gives you the same answer you would get by multiplying or dividing by 10.

to multiply by ten, you move the decimal to the right one space.
to divide you move it to the left.
for any other multiple of ten you move it the same number of spaces that there are zeros in that number ( 10 = 1, 100 = 2, etc)

for example:
2.0 x 10 = 20
in the same way:
2.3 x 10 = 23

100 / 10 = 10
in the same way:
120 / 100 = 1.2
in the same way:
7.8 / 10 = .78

Answer 5

8.7 x 100 = 870. The decimal point moves no. of times to the right side as per the no. of zeroes (Multiplication).

7.8 / 10 = 0.78. The decimal point moves no. times to the left side as per the no. of zeroes (Division).

Answer 6

in multiplication, for every 0, move the decimal right one spot, i.e.

8.7*100 = (move the decimal right 2 spots for 2 0's) 870.

in division, for every 0, move the decimal left one spot, i.e.

7.8/10 = (move the decimal left 1 spot for 1 0) 0.78