Top question:
2.4 and 3.6 can both have 1.2 as a factor. We can factor 1.2 out, and the fraction becomes 2/3. This is the simplest form.
Bottom question:
In order to find the common denominator, we look for the least common multiple. In this case, the least common multiple is 6.3x, so that is the common denominator.
In order to solve this proportion, we cross multiply.
Next, we divide both sides by 4.5 to get x by itself.
x = 1.26 completes this proportion because the ratio of 0.9:x is the same as 4.5:6.3