Minimum attractive rate of return problem (MARR)

Hell everyone,

Im having some trouble with the following problem:

http://img594.imageshack.us/img594/8233/unledzh.jpg

I understand that minimal attractive rate of return is always greater then the cost of capital. What i do not follow in this question is the part where the capital funds are used to fund 25% of all capital projects and how I am supposed to calculate the MARR. (i know the MARR is the weighted average cost of capital, but im not sure how to calculate it)

I need some guidance for this problem, i would like to learn how to approach it.

Thank you for your help.