book Maths Puzzle Numerical Ability

can you please help me with {x+12y=68
{x=8y-12

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6. If 6 men can do a piece of work in 14 days, how many men are needed to do the work in 21 days?
a. 3 b. 9 c. 5 d. 4 e. none
The answer on the answer key is D, but I have no clue how to get the same. Please help! Prizes are often not "worth" as much as claimed. What is the real value of a prize of $5,000,000 which is to be received in equal payments over 20 years, with the first payment beginning today? Assume an interest rate of 5% over the 20 years.