exam Maths Puzzle Numerical Ability

can someone please explain how to do this??!?! I dont care for the answers I just want to understand how to deal with this!
thanks in advance..
Determine S20 for the arithmetic s

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exam Other Question
Write ((81^x+1)(1/9^4-x)) / 27(243)(^x-2) as a single power. If g(x) is an inverse of f(x), then how do the ordinate and abscissa coordinates of the points on f(x) map onto coordinates of g(x)?