300 million test kits are sold this year, if there is 115% increase from last year, how many test kits were sold last year?

Question

jamesj · Answer

Suppose X were sold last year and Y this year.  Then the number
$$ \frac{Y - X}{X} = \frac{Y}{X} - 1 $$
is the percentage increase.

Now if the number has increased by 115% that means that number is 1.15.  You know the number X.  Solve for Y.

jamesj · Answer

$$ \frac{Y}{X} - 1 = 1.15 $$
or 
$$ \frac{Y}{X} = 2.25 $$

jamesj · Answer

*correction: you know the number Y, the number sold THIS year.  You want to solve for X, the number sold last year.

jamesj · Answer

Make sense?

campbell_st · Answer

just work with 115% = 300000 the 1% = 300000/115   then multiply by 100 to get the correct answer 260 870.

jamesj · Answer

That's not the correct answer.

campbell_st · Answer

quick question... how does X/Y - 1 = 1.15 then become X/Y = 2.25... I think that gives a wrong answer...

campbell_st · Answer

and geee...add 3 zeros to my answer... to get it correct