A canister contains two and a half cups of flour.  Greg and Sally have a brownie recipe which calls for one and one third cups of flour.  Greg and Sally want to make one and a half recipes of brownies.  To the nearest whole percent, what percent of the flour in the canister would they use?
1. First, determine the total amount of flour in the canister. The canister contains two and a half cups of flour, which can be written as:
   \[
   2.5 \text{ cups}
   \]

2. Next, calculate the amount of flour needed for one and a half recipes. One recipe requires one and one third cups of flour, which can be written as:
   \[
   \frac{4}{3} \text{ cups}
   \]

3. To find the amount of flour needed for one and a half recipes, multiply the amount of flour needed for one recipe by 1.5:
   \[
   1.5 \times \frac{4}{3} = \frac{3}{2} \times \frac{4}{3} = 2 \text{ cups}
   \]

4. Now, determine the fraction of the total flour in the canister that will be used. The total flour used is 2 cups, and the total flour available is 2.5 cups:
   \[
   \frac{2}{2.5} = \frac{2}{2.5} = \frac{2}{\frac{5}{2}} = 2 \times \frac{2}{5} = \frac{4}{5}
   \]

5. Convert the fraction to a percentage by multiplying by 100:
   \[
   \frac{4}{5} \times 100 = 80\%
   \]

Therefore, to the nearest whole percent, Greg and Sally would use 80% of the flour in the canister.

The final answer is $\boxed{80\%}$