To find out how many cups of sugar you will need when you change the recipe to make it 2 1/2 times as much, you can multiply the original amount of sugar by 2.5.
The original amount of sugar is 1 and 2/3 cups. First, convert 1 and 2/3 to an improper fraction:
1 and 2/3 = \( \frac{5}{3} \) cups.
Now, multiply it by 2.5:
\[ \frac{5}{3} \times 2.5 = \frac{5}{3} \times \frac{25}{10} = \frac{5 \times 25}{3 \times 10} = \frac{125}{30}. \]
Now simplify \( \frac{125}{30} \):
\[ \frac{125 \div 5}{30 \div 5} = \frac{25}{6}. \]
Next, convert \( \frac{25}{6} \) into a mixed number:
25 divided by 6 is 4 (6 goes into 25 four times) with a remainder of 1. So:
\[ \frac{25}{6} = 4 \frac{1}{6}. \]
Thus, if you are making the recipe 2 1/2 times larger, you will need 4 and 1/6 cups of sugar.