Please explain how you answered these questions! I'm quite stuck on the process.

7. Joshua's preferences over cookies (C) and donuts (D) are represented by the utility function U(C, D) = C' + D', MUc = 20, MUD = 2D If the price of a cookies is $1 and the price of a donuts is $2, and Joshua can spend a combined total of $10 on these goods, find Joshua's optimal consumption bundle. (a) C =4, D=2 (b) C =5, D=0 (c) C =0, D = 10 (d) C =5, D = 10 (e) None of the above.8. Bob's preferences for water (w) and coke (c) can be described by utility function U(w, c) = In(w) +c, MUw - -, MU. =1. If the price of a water is $1 and the price of a coke is $2, and Bob has $4, find Bob's optimal consumption bundle. (a) w = 2, c=1 (b) w =4, c=0 (c) w =0, c=2 (d) w =4, c= 2 (e) None of the above.9. Bob's preferences for water (w) and coke (c) can be described by utility function U(w, c) = In(w) +c, MU. = 1, MU. =1. But now the price of Coke suddenly increased to $8 per can. If the price of a water is $1 and the price of a coke is $8, and Bob has $4, find Bob's optimal consumption bundle. (a) w=8, c= - (b) w =4, c= 0 (c) w =0, c= 2 (d) w =4, c= 2 (e) None of the above.2. Andrew's preferences for doing research and sleeping is U(x, y) =6 . VS + R, MUs = #, MUR = 1, where R is number of hours spent on research and S is number of hours spent on sleep. Opportunity cost of sleeping and doing research is the same and equal 1 hour (ps = PR = 1). What's Andrew's optimal schedule if there is only 24 hours in a day (I = 24) (a) S=0, R=0 (b) S =9, R = 15 (c) S = 24, R=0 (d) S =6, R=18 (e) None of the above