How many words to add thanks in advance, this is beyond me.
I have to add keywords to internet content for SEO purposes.
At the moment the current word count is 408. The word fertility is used 27 times.
The new page has 582 words and fertility is used 37 times.
My question is twofold, 1 how do I work this out as a percentage (27 of 408 and 37 of 582)? and 2, how many more times do I have to write fertility in the new content for it to be a similar percentage of the current content.
I've baffled myself just writing it. Thanks to anyone that replies.
 A: Percent is just figure out what proportion of $100$ your quantity is. What's $50\%$ of 30? It's whatever value over $30$ is equal in proportion to $50$ over $100$. So in the first case, you want to solve for 
$\frac{27}{408} = \frac{x_1}{100}$ and in the other case $\frac{37}{582} = \frac{x_2}{100}$. Contrary to the other answer, percent is not always the smaller number divided by the bigger number. For example, $150$ is $150\% $ of $100$, not $66.7\%$. 
To figure out how to get the percents equal to the first case, you solve
$\frac{37+n}{582 + n} = \frac{x_1}{100} = \frac{27}{408}$
because if you add only the word fertility $n$ times to your new essay, you'll have the 37 old ones plus the $n$ new fertility. The total number of words is similarly $582 + n$. 
If it turns out that $n$ is negative when you solve this, that means the percent of fertility in the new essay is actually more than the percent in the old one! This means you'd need to remove fertility a few times to make the percents equal. 
If this is still confusing to you, please do not hesitate to ask. 
