exam Maths Puzzle Numerical Ability

I need help on this homework problem. Thank you for any help you can provide.

a1=1 r=-1 find s20

I keep getting 1 as the answer but the answer is 0. what am I missing?

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exam Other Question
1.) If one-half of one interger is subtracted from three-fifths of the next consecutive interger, the difference is 3. What are the two intergers?

2.)Passanger train can travel Please help me solve the problem below by addition, substituting and graphing...

4x - 6y = 12
2x + 2y = 6

Thank you for your help