Yolanda initially owns 4 rabbits.
For year 0 she has 4 rabbits.
Year 1 = 4*2 = 4*2¹= 8 since it doubles after every year, it will be multiplied by 2
Year 2 = 4*2*2 = 4*2² = 16
Year 3 = 4*2*2*2 = 4*2³ = 32
Year 4 = 4*2*2*2*2 = 4*2⁴ = 64
This is a kind of Geometric Progression.
Year n = 4*(2ⁿ)