So if you know that a7 is 31 and a15 is 63, then all you have to do is find how many sequences come in between the two and divide the difference to find the rate of change.
So, 15-7=8, there are 8 steps before a7 reaches a15.
Next, you subtract 63-31 to find how much is separating the two. So 63-31= 32.
Finally, you divide 32 by 8 to get 4 because that’s the rate of change. So every time the sequence goes up by 4.
Since a15 is 63 all you have to do is multiply 4 times 4 because a15 is 4 sequence numbers away from a19. And so 63+ 16 (4*4)= 79.
