1) If x and y are the two digits f the number 653xy such that this number is divisible by 80, then x+y is equal to:

(i) 2
(ii) 3
(iii) 4
(iv) 6

Answer is :

Since 653xy is divisible by 2 as well as by 5, so y = 0
Now 653x0 is divisible by 8 so 3x0 is also divisible by 8.
By hit and trial x=6 and x+y = 6

2) The smallest number which when diminished by 3 is divisible by 21,28,36 and 45 is...

(i) 869
(ii) 859
(iii) 4320
(iv) 427

Answer is The required number = l.c.m. of (21,28,36 ,45)+3=1263

3) If 1.5x=0.04y then the value of (y-x)/(y+x) is

(i) 730/77
(ii) 73/77
(iii) 7.3/77
(iv) None
Answer  is: x/y = 0.04/1.5 = 2/75
So (y-x)/(y+x) = (1 - x/y)/(1 + x/y) = (1 - 2/75)/ (1 + 2/75) = 73/77

4) The average age of a class is 15.8 years. The average age of boys in the class is 16.4 years while that of girls is 15.4 years. What is the ratio of boys to girls in the class?

(i) 1:2
(ii) 3:4
(iii) 3:5
(iv) None of these

Answer is :
Let the ratio be k:1.
Then k*16.4+1*15.4 + (k+1)*15.8 (16.4-15.8)k = 15.8 - 15.4 k=0.4/0.6 = 2/3 so required ratio = 2:3