An outgoing batch of students wants to gift a PA system worth Rs 4,200 to their school. If the teachers, offer to pay 50 more than the students and an external benefactor gives three times the teacher's contribution, then how much should the teachers donate?  Choose the correct choice from the following answers:
A) Rs 600
B) Rs 900
C) Rs 700
D) Rs 1200
E) None of these
Explanation :   The ratio of the share students : teacher: benefactor=1:1.5:4.5.   So the proportion to teacher's share = 1.5/7.   Hence, the teachers would donate (1.5/7)×4200=Rs 900.   Answer : Rs 900
<result>B</result>
An outgoing batch of students wants to gift a PA system worth Rs 4,200 to their school. If the teachers, offer to pay 50 more than the students and an external benefactor gives three times the teacher's contribution, then how much should the teachers donate?