Use this equation y = a (1 - b)^x
Where a is the initial population and b is the decay factor: 40/100 = 0.4
Therefore, the answer is that the decay factor b is 0.4.
This table shows some figures using the decay factor.
year decrease population
0 0 a(1 - 0.4)^0 = a
1 a*0.4 a (1 - 0.4)^1 = a*0.6 = 0.6a
2 0.6a*0.4 a(1 - 0.4)^2 = a(0.6)^2 = 0.36a
3 0.36a*0.4 a(1 -0.4)^3= a(0.6)^3 = 0.216a