What approximate value should come at place of the question mark(?) in the following question?

Directions: Each of the questions consists of a question followed by three statements. You have to study the questions and the statements and decide which of the statements(s) is/are necessary to answer the question. 

What is the area of the hall?
 I. Material cost of flooring per sq. meter is Rs. 250
 II. labor cost of flooring the hall is Rs. 3,500
 III. Total cost of flooring the hall is Rs. 14,500

Income of A and B are in ratio 3:4 and their expenditure are in ratio 4:7 their saving are equal if A save Rs 1760 more and B increase his expenditure by Rs 880 then their saving are in ratio 7:4 .find the average income of A and B?

Direction (66-70): What should come in place of the question mark(?) in the following number series?

4, 5, 14, 51, ?

One boy from a group of eight boys left and a new boy with a weight of 56 kg joined the group, which resulted in an average weight of 2.5 kg has increased, what is the weight of the new boy?

Rahul and Rohan enter into a partnership with   their capitals in the ratio 7:9. At the end of 8^th month, Rahul  withdraws his capital. If they receive the  profits in the ratio 8:9, find how long Rohan’s capital was used.

Mr. Girdhar spends 50 % of his monthly income on household items and out of the remaining he spends 50% on transport 25% on entertainment, 10% on sports and remaining amount of Rs.900 is saved. What is Mr.Giridhar's monthly income?