The population is multiplied by 1.15 in a decade. After 4 decades it will be multiplied by 1.15^4, so will be 875 persons.
In terms of persons per year, you have
.. p(t) = 500*1.15^(t/10)
.. p'(t) = 500*(1/10)*ln(1.15)*1.15^(t/10) ≈ 6.988097*1.15^(t/10)
so
.. p'(40) = 12.22 . . . . persons/yr
