the answer is W=10.5 but i don't understand why. that is all the information i am given

Alternative A "You consider investing your wealth of 10 TMU in shares, with the probability of 50% of your wealth being 15 TMU and 50% of being only 9 Alternative B "Alternatively, you consider an investment of your wealth of 10 TMU in a savings account that guarantees a wealth of W TMU after one year." TMU after one year." How large must W be so that A and B are equivalent to you? Alternative A "You consider investing your wealth of 10 TMU in shares, with the probability of 50% of your wealth being 15 TMU and 50% of being only 9 Alternative B "Alternatively, you consider an investment of your wealth of 10 TMU in a savings account that guarantees a wealth of W TMU after one year." TMU after one year." How large must W be so that A and B are equivalent to you