book Maths Puzzle Numerical Ability

Can somone answer this problem?
The number of permutations of two items from a data set is always two times the number of combinations when taking two objects at a time from the same data set
Explain why this statement is true or false

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book Other Question

The formula is 1020 squre root of P(1-.01square root of P)=G. P is the population in thousands,this formula is the amount of water in gallons per minute the city needs.
A city Malik has $3.08 in dimes, pennies,and nickels. He has nine fewer nickels than dimes and one more peeny than dimes. How manof each coin does he have.