# Math Help - If 200 of something costs £53.80. then how much does 20 of the same thing cost?

1. ## If 200 of something costs £53.80. then how much does 20 of the same thing cost?

OK this is dumb beyond belief, but I have been offered a cheap deal on something that seems too good to be true. Basically for each pack of 200 I pay £53.80, but I want to know how much 20 costs. To do this I assumed all I had to do was divide 200 by 53.80 to get the price for one of these items and then multiply this by 20 to get the cost of 20. But this works out at a stupidly cheap price of about 73 pence for 20. I know this is dumb and it can't possibly be right, as I have never seen these items sold so cheaply. What am I doing wrong?

2. ## Re: If 200 of something costs £53.80. then how much does 20 of the same thing cost?

Originally Posted by jebus197
divide 200 by 53.80 to get the price for one of these items
Wrong way. If the unit cost is $u$, we have $200u=53.80\Rightarrow u=\frac{53.80}{200}=0.269$.

So $20u = 20\cdot0.269=\pounds5.38$.

Another way of looking at it is to note that 20 is 10% of 200. So 20 units should cost 10% of 53.80, which would be 5.38.

3. ## Re: If 200 of something costs £53.80. then how much does 20 of the same thing cost?

Thanks, seems obvious now, lol.

4. ## Re: If 200 of something costs £53.80. then how much does 20 of the same thing cost?

Great! it's simple, clear. Thanks for sharing

5. ## Re: If 200 of something costs £53.80. then how much does 20 of the same thing cost?

first we will take out the cost of one piece such as £53.80/200 = £0.269
so then 20 piece cost will be £0.269 * 20 = £5.38

6. ## Re: If 200 of something costs £53.80. then how much does 20 of the same thing cost?

Originally Posted by kalwin
first we will take out the cost of one piece such as £53.80/200 = £0.269
so then 20 piece cost will be £0.269 * 20 = £5.38
If you want to do it the hard way! Much simpler is to note that 20 is one-tenth of 200 so the cost of 20 is one-tenth the cost of 200. One-tenth of £53.80 is £5.38.