This is an example of a Geometric sequence:

2, 4, 8, 16, 32, 64,…

The common ratio, which is what the previous number was multiplied in the sequence is 2 in this example because every number gets multiplied by 2.

To find r you need to divide a term by the preceding term. For the example above you could find r by doing this: ÷ which would equal 4 ÷ 2 which equals 2. So r=2.

Now to find you would need to the formula of x

So for this example, would be 50 x which equals 562 949 953 421 312.