In a class, if 4 students sit in each bench, 3 benches remain empty. But if 3 students sit in each bench, 6 students do not get any seat. Find the total number of students. 

The ratio of prices of two houses was 16:23. Two years later when the prices of the first had risen by 10% and that of second by Tk. 477, the ratio of their prices become 11:20. Find the original prices of the two houses.

Twenty years ago, the father's age was 4 times the age of his son, 4 years hence the father's age  2 times the age of his son. Find the present age of the father and the son.