If the time increases by 7 years, then simple interest increases by Rs 8400 on a sum of Rs 6000. What is the rate (in percentage) of interest per annum?
The average marks obtained by Raghav in 12 tests is 24. Zubeida has maintained an average of 23 so far but has taken only 9 of the tests. If each test is out of 30, at least how much Zubeida score in any one of the remaining three tests to still have a chance to match Raghav's performance?