I don't think this is an exponential decay.
First, x is incremented by same amount, 10 years, each step. However, y is not reduced by the same factor.
25165824 / 393216 = 64 but
393216 / 12288 = 32
showing that y is not exponential.
Also, if it was some kind of multiplicative reduction, y would never reach 0.
Is there any more information for the problem?