So George rode two days, we want to know the total amount of miles he rode, adding right? The key to adding fractions is creating a like denominator. Here, that can be 4 because we can simplify 12/8 to 6/4. 6/4 + 1/4 = 7/4 miles. You can plug this into your calculator to get a decimal of 1.75.
Or, since this problem didn't require the answer to be a fraction, you could have divided both fractions to begin with and added them in your calculator. 1.5 + 0.25 still equals 1.75.
