Math Is Fun Forum

jmoore

Member

Registered: 2008-01-09

Posts: 25

Multiply decimals by whol numbers?

Hey all

I am trying to learn to multiply whole numbers by decimals so that I can work out percentages, I found this example in a book

"find 15% of 60 I understand how to convert it to decimal 15x100 is 0.15 and then it says 0.15x60 =9 but I dont understand where they got the nine from i tried multiplying 15x60 and i got 135 add the decimal point in and i get 1.35 but still no 9, if i can understand how this was done it would make my life that little easier.

thanks all

Joe

JaneFairfax

Member

Registered: 2007-02-23

Posts: 6,868

Re: Multiply decimals by whol numbers?

You shifted the decimal point correctly. Where you went wrong was on the multiplication. 15 × 60 is not 135. It should be 900. Check your working, or try the multipication again..

jmoore

Member

Registered: 2008-01-09

Posts: 25

Re: Multiply decimals by whol numbers?

OK so for this example

"An Investor withdraws 40% of her saving from her account wich holds £800. how much is left"

so i would convert the 40% to decimal 0.40 then times it by 800 which is the same as 40x800 which is 3200 put the decimal point back in get 320 so the answer is 320 am i right?

Daniel123

Member

Registered: 2007-05-23

Posts: 663

Re: Multiply decimals by whol numbers?

Almost. The question says "how much is left?", so you have to take that 320 away from 800.

You obviously didn't notice this part of the question. If you had, it would have been quicker to calculate 60% of 800 (as this would be the amount left after removing 40%)

jmoore

Member

Registered: 2008-01-09

Posts: 25

Re: Multiply decimals by whol numbers?

Sorry guys one more question

"A health club raises its annual subscription of £240 by 15% what will the new subscription cost?" when i work it out i get £360
240x15 =360 now when i look at the answer it has £260 are they wrong as i cant understand if they are right where i got the extra 100 from

Daniel123

Member

Registered: 2007-05-23

Posts: 663

Re: Multiply decimals by whol numbers?

Both answers are incorrect.

15% of 240 = 15 x 240 / 100 = 36

If the subscription is being raised by 15%, it is now 15% more than it was before. This means it is now £36 more than before, so it is now £240 + £36 =£276

Another way to look at it: it is now 115% of what it was before. 240 x 115 / 100 = 276

Last edited by Daniel123 (2008-03-22 03:47:59)

jmoore

Member

Registered: 2008-01-09

Posts: 25

Re: Multiply decimals by whol numbers?

thanks guys/gals

John E. Franklin

Member

Registered: 2005-08-29

Posts: 3,588

Re: Multiply decimals by whol numbers?

4 x 8 = 32
4 x 80 = 320
40 x 80 = 3200
40 x 800 = 32000  See?
32000.% = 320.00
32987% = 329.87

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