Answer: 87.5%
Explanation:
Value of a Customer = Margin * Retention rate / (1 + Annual discount rate - Retention rate)
We shall assume a margin of 1 so no need to include it:
= 75% / ( 1 + 5% - 75%)
= 2.5
This value needs to double so assume the retention rate to double this is x and use the formula above to find it:
5 = x / (1 + 5% - x)
5 * (1 + 5% - x) = x
5 + 0.25 - 5x = x
5.25 = 5x+ x
6x = 5.25
x = 5.25 / 6
x = 87.5%