Answer: 30 dominoes in total.
Explanation:
Each line is 21 inches long.
the space between dominoes is 1 + 1/2 inches.
(assuming that the distance includes the domino itself) Now, we need at least two dominoes to see this distance,
The we have N + 1 dominoes per line, and the distance is:
N*(1 + 1/2)inches = 21 inches
If we find N, we can find the number of domineoes in each line:
N = 21/(1 + 1/2) = 14
This means that we have 14 + 1 = 15 dominoes in each line, and we have two lines, so we have 30 dominoes in total.