How does Casio 99 1ES calculator store a number and input only one number, such as 80?

By default, the 99 1es system has 9 variables (a, b, c, d, e, f, x, y, m).

1. The simplest thing is to store the number in m, and just enter 80-m+. Just press RCL+M+ when querying. But once you press another number, such as 5, and then press M+, the value of m will change.

2. The insurance method is stored in A, B, C, D, E and F (X and Y will change when Pol and Rec are used). Method:

Enter 80, shfit+RCL+ (select the stored letters, where A corresponds to (-) B. ,,,, omitted later)