Answer:
if he sells 12 every 8 months, supposing the trend continues, in 34 months he’ll sell...
12(properties) = 8 months
12/8 = 1 month
1 month = 1.5(properties)
2 months = 3 properties
(since 1.5 properties is unrealistic, but it helps in the math, to find for one month)
so for 34 months he’ll sell...
1.5*34 = 51 properties