In a class of 60 where girls are twice to that of boys. Annu , a girl 20th from the top. If there are  12 girls ahead of her then, how many boys are after her? 

13

Number of girls are twice to that of boys. 

i.e.   G = 2B 

Total number of students = 60

 G + B = 60

 2B + B = 60 

 3B = 60 

B= 20 ( number of boys) 

 G= 40 ( number of girls ) 

Now, 

Annu is 20th from top and 12 girls are ahead of her . Till 20 there are 13 girls .

That means there are 7 boys ahead of her . 

So, number of boys after Annu = total number of boys - number of boys ahead of her 

                                               = 20 - 7 = 13