what is my cap rate 24 % or 37%

[Ryan Scritchlow]

I am purchasing a property that has 3 units that will bring in 1800 per month total. It is a forclosure and needs a little work but after I do the repairs I will have 45 k invested. Im paying cash I used the caculator that is provided and it came up with a 37% cap rate and when I run the 50% rule as I under stand it from reading it comes to 24% is there really this much varence between useing the 50% rule and actualy running the numbers?
Thanks