<p>So I was taught that in SAT Math portion, y(x) = y1(x1) is better to use because it's faster and more efficient. This is for directly proportional. For inversely proportional, I was taught to use the formula y/x = y1/x1. So far, this formula has worked for many of the easy to medium problems and a lot of the difficult problems. </p>
<p>Only twice has these two formulas failed me, both in the difficult range. Like for instance the problem in Official SAT study guide test #8 section 2 #7: If y is inversely proportional to x and y=15 when x=5, what is the value of y when x=25? </p>
<p>When I solved with the formula I was taught, I get y=75. And this sounds right because y is greater than x. However, I got that wrong and I solved it the other way, the way high school teachers teach, with y=k/x. I got the answer right by solving k. </p>
<p>Did I miss something or is y/x = y1/x1 and yx = y1x1 a bad formula to use in SAT?</p>
<p>I’m not sure of the source of the formula that you are using.</p>
<p>Think of the problem as a “word” problem.</p>
<p>If y is directly proportional to x, then y is a constant times x. That’s to say that when you double x you double y, and when you triple x you triple y, etc. You write this as: y = kx,</p>
<p>If y is inversely proportional to x then y is a constant times 1/x. The word “inverse” is to suggest “1/x”. Here when you double x, you half y, when you triple x you have 1/3 of y, etc. You write this as y = k/x.</p>
<p>And this “word” description of the relationship between two variables can be still more complex, as for example y is directly proportional to the square of x. Here you have y = kx^2.</p>
<p>In all these relationships think of “is” as an equal sign. Perhaps that will help you remember how to proceed.</p>