Brain teaser

Please answer the following questions.

1.It took 20,000 workers to build the Taj Mahal in 20 years. How many workers would be required to build it in 10 years?
 

2. Clark Rupa does 5 sets of pushup, 6 sets of sit-up, 7 sets of squat, and 8 sets of bench dip in a day. So, which organ in his body would be the largest?

3. Which one is correct? "Penguins flies" or "A Penguin flies"

4. Complete the series. 9 = 4, 21 = 9, 22 = 9, 24 = 10, 8 = 5, 7 = 5, 99 = 10, 100 = 7, 16 = ?, 17 = ?