To solve this problem, we need to first simplify the expression inside the parentheses, then divide the result by 0.8.

First, let's work out the expression inside the parentheses:
(3 2/3 + 1 3/4) : (67/12 - 21/4)
Convert the mixed numbers to improper fractions:
(11/3 + 7/4) : (67/12 - 21/4)
Find a common denominator:
(44/12 + 21/12) : (67/12 - 63/12)
Combine the fractions:
65/12 : 4/12
Divide the fractions:
65/12 ÷ 4/12 = 65/12 * 12/4 = 65/4 = 16.25

Now, divide the result by 0.8:
16.25 / 0.8 = 20.3125

Therefore, the final answer is 20.3125.