Why do you only need to look at the number of dollars to know that 5.12 is greater than 4.82?

You only need to look at the number of dollars because both values are expressed in the same currency and have the same number of decimal places. When comparing two monetary amounts, the whole dollar amount is the primary factor, and since 5.12 has a greater whole dollar value than 4.82, it is clearly greater. The cents (decimal part) only serve to further differentiate the amounts but do not change the overall comparison.