A proportion is simply a statement that two ratios are equal. It can be written in two ways: as two equal fractions a/b = c/d; or using a colon, a:b = c:d. The following proportion is read as "twenty is to twenty-five as four is to five."
If we multiply the equation x-3y=7 by 3, we get 3x-9y=21, which is also the second equation. That means the two lines are on top of each other, meaning infinite many solutions.
