how can I increase the amount a number decreases over time?

how can I increase the amount a number decreases over time?

Kind of basic, I have a number that starts at a value (say 5000) and I
need it to decrease at an increasing rate as a different number decreases.
as x drops from 1500 to 0 at a steady rate I need y to drop from 5000 to 0
at the steadily increasing rate. first 75% should be very slow then it
should decrease at a exponentially increasing rate.
want to do this in c++ with any math functions available.