The right answer is the constant of proportionality
If two variables are related and this relationship is that there is a<em> constant ratio</em> between them, then we are talking about proportionality. There are two types of proportionality.
1) Direct Proportionality:
In a mathematical language this is given by this equation:
where 
is in fact this constant of proportionality. If 
<em>increases,</em> 
also <em>increases</em> at the same rate. On the other hand, If <em> decreases, </em> also <em>decreases</em> at the same rate.
2) Inverse Proportionality:
In a mathematical language this is given by this equation:
where is also called the constant of proportionality. If <em>increases,</em> <em>decreases</em> at the same rate. On the other hand, If <em>decreases,</em> <em>increases</em> at the same rate.
1) The ratio of used books to all books is 6:14
2) The ratio of used books to new books is 6:8
29-11=18
18²+18²=p²
324+324=p²
p²=648
p=18√2
That's your answer.
