21. 
In a class, there are 18 boys who are over 160 cm tall. If these constitute threefourths of the boys and the total number of boys is twothirds of the total number of students in the class, what is the number of girls in the class ? 


22. 
A father is now three times as old as his son. Five years back, he was four times as old as his son. The age of the son (in years) is 


23. 
A waiter's salary consists of his salary and tips. During one week his tips were 5/4 of his salary. What fraction of his income came from tips ? 


24. 
If you write down all the numbers from 1 to 100, then how many times do you write 3 ? 


25. 
If 100 cats kill 100 mice in 100 days, then 4 cats would kill 4 mice in how many days ? 

