How do you simplify to tell whether the ratios #3/10, 9/20# form a proportion?
A proportion, by definition, means that two ratios are equal.
To check if two ratios are equal, you have to bring both to their simplest form and see if you get the same value.
Let's look at the two ratios we have and see if we can simplify them.
The ratios do not form a proportion.
Probably the quickest check is to cross-multiply.
If the fractions are equivalent, their cross-products are equal.
So the ratios do not form a proportion.
Another check is to divide each fraction to get a decimal, these are easy to compare.
They are not equal.
Changing to a common denominator is another option: