The general form of an exponential equation for growth is
y= ( 1 + r ) ^x
and for decay is
y= ( 1 - r ) ^x
In general, if the number inside the parenthesis (the growth or decay rate) is greater than 1, it's a growth problem. If the number inside the parenthesis is greater than 0 but less than 1 (in other words a positive fraction), it's a decay problem.
