31. 

Among the 50 students in a class, what percentage of students like biology and chemistry?

32. 

For any n observations of data, what is the value of (∑x) - nx ?

33. 

Simplify : 0.728x0.728-0.272x0.272
                               0.456

34. 

If ^x/_y = ³/₅, then 5x + 2y   is equal to
                         5x - 2y

35. 

A person's present age is two-fifth of the age of his mother. After 8 years, he will be one half of his mother at present?

36. 

Find the value of 1.2x1.2x1.2-0.2x0.2x0.2
                            1.2x1.2+1.2x0.2+0.2x0.2

37. 

A bicycle marked at Rs. 1,500 is sold for Rs. 1,350. What is the percentage of discount?

38. 

Which one of the following is the smallest ratio?
7 : 3, 17 : 25, 7 : 15, 15 : 23

39. 

In a mixture of 60 litres, the ratio of milk and water is 2 : 1. If this ratio is to be 1 : 2, then what is the quality of water to be further added?

40. 

Two numbers are in the ratio 3 : 5. If 9 be subtracted from each, then they are in the ratio 12 : 23. Find the second number