3 . A, B and C can do a piece of work in 15 days. All three worked for 2 days and then A left. B and C worked for 10 more days and B left. C worked for another 40 days and completed the work. In how many days can A alone complete the work, if C can complete it in 75 days?
4 . A tower standing on a horizontal plain subtends a certain angle at a point 160 m apart from the foot of the tower. On moving 100 m towards the tower, it is found that the subtended angle is now twice as before. What will be twice the length of the height of the tower?
6 . The average age of a college is 21.8 years. The average age of students of college is 24.2 years and average age of lecturers of college is 20.6 years. Find the ratio of the number of students to that of lecturers?
7 . A man and a woman 81 miles apart from each other, start travelling towards each other at the same time. If the man covers 5 m/hr to the women's 4 m/hr, how far would the woman have travelled when they meet?