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.

Three persons buy cold drinks of different bands A, B, C. The first person buys 12 bottles of A, 5 bottles of B, 3 bottles of C. The second person buys 4 botties of A, 6 bottles of B and 10 bottles of C. The third person buys 6 bottles of A, 7 bottles of B and 9 bottles of C. If each bottle of brand A costs tk. 4, each bottle of B costs tk. 5 and each bottle of C costs 6, then find the total sum of money spent individually by three persons for the purchase of cold drinks.