Question 565392
X=1, 2, 3, 4, 5, 6, 7
Y=1, 3, 7, 15, 31, 63, 127
<pre>
If you look at the Y-values, you will discover that
each one is one less than the corresponding term 
of this sequence:

2, 4, 8, 16, 32, 64, 128

which are the 1st, 2nd, 3rd, 4th, etc. powers of 2.

So the X-th term of the given sequence is 1 less than
2<sup>X</sup>.  So the equation you're looking for is:

Y = 2<sup>X</sup> - 1

You just have to be observant and familiar with the
various powers of small numbers to do problems like these.

Edwin</pre>