Answer:
2/3 quart
Explanation:
The 10 by 12 ft wall has an area of 120 ft^2. The 9 by 10 ft wall has an area of 90 ft^2.
Thus, the following equation of ratios provides us with the answer:
90 (1/2)
--------- = -----------
120 x
or 3/4 = 1/(2x), or
6x = 4. Thus, x = 4/6 or 2/3.
2/3 of a quart would be enough for one coat on the 10 by 12 ft wall. Note that only 1/2 of a quart would suffice for one coat on a 9 by 10 ft wall (for comparison).