please show all solution steps as processes

AND DRAW THE BRANCH AND BOUND TREE CLEARLY

3. [25 pts.] Four projects are available for investment. The projects require the cash flows and yield the net present values (NPV) (in millions) shown in the following table. If 20 million Lira is available for investment at time 0 , find the investment plan that maximizes NPV. All investments are required to be 0 or 1 (fractional investment values are not permitted). a. Formulate the mathematical model. (Write the decision variables, objective function and the constraints.) [10 pts.] b. Find the optimal solution by using Branch and Bound method (Draw the branch and bound tree clearly, write also lower bounds (LB) ) (Left branches xi=0, right branches xi=1 ) [15 pts. ]