The function key program trick for HP15c

f LBL A

STO 1 Stores the value in X in R1

R/S            Stops. Here you can press another function key or R/S to solve for A

 

... some code here to solve for this variable

 

STO 1

g RTN

 

Same with other labels of the first row, each using with another register to store the value: 2, 3, 4, 5.

 

Now, this is typically used for a main program, say TVM; but you can use these labels to enter the variables in registers 1, 2, 3, 4, 5, and then press GSB another label to run other program with solve. In my current set up, I use the "solver loader" in 4 different programs (not all variables - but for sure the first three). When you enter it once in a program in your calculator, you can use it to fill in R1 to R5 in any other program (if these are the ones you set it for). Just make sure of not pressing R/S but the initial label of whatever program you want to run. 

 

A nice trick that saves programming steps and makes an easier number entry for your programs!