You will have to estimate the two fraction by the benchmark which is 0,1/2, or 1 then add the fraction that you estimate with the mixed number beside it. Add the to number together to get the answer.
Example:
3 2/3 - 1 1/2
3 2/3 = 4
1 1/2 = 1 1/2
4 - 1 1/2 = 2 1/2
