Let a be x days and b be 1/24- 1/X/ day. 20 times1/24+(1/24-1/x) times 20= 1. The second question is difficult to type. You can use the number x to represent the money of the first two kinds, and then use inequality to solve the second kind, and get that the first kind of plan is greater than the second kind of plan or the second kind of exam in that range.