Final answer:
Theo added 1/12 cup less than a full cup of water to the pancake recipe.
Step-by-step explanation:
To find out how much less than a cup Theo added, we need to subtract the actual amount of water added from a cup.
Theo added two-thirds of a cup of water, which is equivalent to 2/3 cup. Then, he added an additional quarter cup of water, which is equivalent to 1/4 cup. So, the total amount of water added is 2/3 + 1/4 = 8/12 + 3/12 = 11/12 cup.
To find out how much less than a cup Theo added, we subtract 11/12 cup from 1 cup:
1 - 11/12 = 12/12 - 11/12 = 1/12 cup.
Therefore, Theo added 1/12 cup less than a full cup of water to the pancake recipe.