There were 64 houses, last month, 1/4 of the houses were sold, this month 3/4 of the houses were sold, how many houses are left to be sold?

You start with 64 houses and sell 1/4 of them ...

(1/4) x 64  =  16

So, 16 houses were sold.

This means that 64 - 16 = 48 houses are still left to be sold.

This month, 3/4 of these houses were sold.

(3/4) x 48  =  36

So, 36 houses were sold this month.

This means that 48 - 36 = 12 houses that remain to be sold.

You start off by subracting one fourth of the houses so 3/4 of the houses are left. 3/4 of 3/4 is 9/16. 3/4 - 9/16 = 3/16. 3/16 of 64 is 12.