He should use 886 fluid ounces of soap.
Step-by-step explanation:
Know that 8 fluid ounces make 1 cup.
Now 24 cups need 32 fluid ounces or, 3 fluid ounces of water need 32 fluid ounces of soap.
Thus, by unitary method,
84 ounces of water need 32 × 84 / 3 ounces of soap.
Thus the answer comes to 886 ounces.