Question 1203595
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Notice that the numbers 1,4,7,10,13 form an arithmetic progression with the common difference of 3:

    4-1 = 3;  7-4 = 3;  10-7 = 3;  13-10 = 3.


So, you can use the standard formula for the n-th term of an AP

    Y = 1 + 3*(X-1),  which is the same as  Y = 3X -2,  X = 1,2,3,4,5.


Or, if you don't know about arithmetic progression, you can guess the formula
and check its validity directly on your own.
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Solved.


Enjoy and have fun.