Cheng has $21000 in his savings account today. He earns interest at 4% p.a. effective. He plans to make the following withdrawals from his account $7200 in 3 years and $6800 in 8 years. He will still have $X left in his bank account right after the last withdrawal. Which one(s) of the following equations can be used to find $X? (There may be more than one correct answer. You will lose mark by picking a wrong answer. The minimum mark for the question is zero.) Select one or more: a. X = 21000 + (1+0.04)8 7200 *(1 + 0.04)3 6800(1+0.04) b. X = 21000 7200*(1 + 0.04)3 6800 + (1 + 0.04) c. 21000 7200*(1 + 0.04)3 +6800 *(1 + 0.04)-8 + X *(1 + 0.04)-8 d. X = 21000 7200*(1 + 0.04)5 6800 e. X = 21000 *(1 + 0.04)8 7200 *(1 + 0.04)5 6800 f. None of the equations can be used to find $X