Answer:
24 pounds of flour.
Explanation:
In the problem, the unknown quantity is the supply, which will be defined as
. So, we know that Adam used 1/8 and 3/4 of the supply, and he used a total of 21 pounds of flour, the equation to represent this is:

Now, we solve for
, to find the supply:

Therefore, the supply was 24 pounds of flour.