Work out improper fractions by multiplying the denominator by the whole number then adding the numerator. This numerator divided by the denominator is the improper fraction. Or, take the whole number, work out how many fractions it's equivalent to. So if the fraction denominator is 5, for example, then whole number 1 is 5 fifths or 5/5; the whole number 3 is 3*5=15 fifths, so 3=15/5, and so on.
66 1/3=66+1/3; 66*3=198 so 66=198/3, add on another third makes 199/3. Divide by 100: 199/300.
66 3/5=66+3/5; 66=330/5, add on 3/5 makes 333/5. Divide by 100: 333/500.
66+2/3=198/3+2/3=200/3. Divide by 100: 2/3. Correct!
66+7/10=660/10+7/10=667/10. Divide by 100: 667/1000.