The numbers represent the
amount of money you get back in each of the "failure" and "success" states, if you invest a*w0 in A and (1-a)*w0 in B.
The terms in the first curly bracket represents the money obtained in the failure state. If you invest a*w0 in A, which produces a return of -100% (
ie you lose all your money), then you end up with 0. If you invest (1-a)*w0 in B, which produces a return of +50%, then you end up with your original investment plus 50% extra,
ie 1.5*(1-a)*w0. So, your total wealth in the failure state is 0 + 1.5*(1-a)*w0.
The second curly bracket is the money obtained in the success state. If you invest a*w0 in A, which produces a return of +300% (
ie you get your original investment back, plus a profit equal to three times that amount), then you end up with 4*a*w0. If you invest (1-a)*w0 in B, which produces a return of +50%, then you again end up with 1.5*(1-a)*w0. So, your total wealth in the failure state is 4*a*w0 + 1.5*(1-a)*w0.
Hope this makes sense.
