You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.

Question: A train running at the speed of 60 km/hr crosses a pole in 21 seconds. What is the length of the train?

 Option A: 350 m

 Option B: 278 m

 Option C: 876 m

 Option D: 150 m

 Option E: 267 m
Option A

Question: Two pipes A&B can fill a tank in 24 min and 32 min respectively. If both the pipes are opened simultaneously, after how much time B should be closed so that the tank is full in 18 min.

 Option A: 8 min

 Option B: 3 min

 Option C: 21 min

 Option D: 23 min

 Option E: 24 min
Option A

Question: A sum of money invested at compound interest amounts to Rs.800 in 3 years and to Rs.840 in 4 years. The rate of interest per annum is

 Option A: 2x1/2%

 Option B: 4%

 Option C: 5%

 Option D: 6x2/3%

 Option E: None

Option C