1. The title of this kind of problem can be analyzed as follows: since half of the mobile phones sold the next day were less than five, there was still 18. If the number of mobile phones sold the next day is X, the following equation should hold.

(x/2-5)+ 18=x

x/2+ 13=x

x/2=x- 13

x=2x-26，

X=26 (unit)

Knowing that there are 26 mobile phones the next day, you can set the mobile phone on the first day as Y, and the following equation should also be established.

y/2+20+26=y

y/2=y-46

y=2y-92

Y=92 (unit)

The following topics can be analyzed from the back with reference to the above analysis.