The software will “pay for itself” if it is used 17 months or longer. This can be calculated by finding the difference of the build and buy, which is $33,000. Then you find the difference of the monthly fees, which is $1,938. Divide the $33,000 by $1,938 and this tells you how long the vendor’s solution will be valid. If the software is to be kept longer than 17 months then it’d be more cost effective to build the software than to buy it. 1 is incorrect because after 17 months the build solution is a better choice. 3 is incorrect because the question already explains that the per month transaction will be more than 30,000. 4 is incorrect because this is not a valid choice (especially for the vendor).