One of the most important tips I can offer on Data Sufficiency is to pay attention when the GMAT gives you a bit of information without calling much attention to it.
In this case, I am honing in on the fact that statement 1 talks about the "average" returns each person received.
To establish a lack of sufficiency, because the answer is E, we need to create a situation in which Leslie makes more and a situation in which Kerri makes more.
Scenario 1 - Leslie makes more
Leslie starts with $100 and makes 20% both years.
100(1.2)(1.2) = 144
Kerri starts with $200 and makes %5 both years.
200(1.05)(1.05) = 220.50
If we can come up with a scenario in which Leslie makes less that $20.50, then the answer must be E.
Scenario 2 - Kerri makes more
Leslie starts with $100. She loses 50% the first year and gains 90% the second year. We can do this, because the average of (-50) and (90) is 20.
100(.5)(1.9)= 95
In this scenario, she did average +20% returns, but she actually lost $5 over the two years.
The answer is E
