book Maths Puzzle Numerical Ability

I dont even know where to start the question is
Find the common difference of the arithmetic sequence:
1/2, 1/6, -1/6, -1/2

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book Other Question

A number t increased by the sum of seven and the square of another number s is ten

How can i write this question so that it reads as an equation? "1/2 the difference of a number and 15"