VAT CALCULATOR VERSION 2 (2K ZX81)
from ZX Computing 2, August/September 1982, page 110

This next program is a version that allows the user to specify the VAT rate at
the outset and to change it during the running of the program if desired, and
the program also has a scrolling display. When running the program, you will
get a prompt to enter the VAT rate.

If it is 15%, enter 15 (NEWLINE). Do not enter the word PERCENT or a 0/0 symbol
as this will cause an error. The computer will then tell you how to change the
VAT rate if you wish to change the rate. To do this, you enter the letter Z
followed by NEWLINE. You may expect this to cause an error since the computer
is expecting a numeric INPUT (line 110). However, Z has previously been defined
as a variable (line 10), and B will take the value of Z, in this case a very
small number, 1E-9 which would never be entered in the normal usage of this
program. So if you enter either 1E-9 or Z the program recognises this as a
signal that you want to change the VAT rate and takes appropriate action. The
thing to note about entering the amount to be evaluated is that you should
enter the amount in pounds, without the  symbol, eg. twenty pounds would be
entered as 20, twenty pounds and twenty three pence would be 20.23.

It does not matter in the least if you enter any trailing spaces, eg. 20.00
since this will not affect the arithmetic, and all numbers printed are
justified to two places of decimal by the subroutine starting at line 270. This
program requires 2K to run. The load and go routine at line 9900 is very
useful, because the program runs automatically when loaded from tape and is in
the mood of "handy utility" that the program was intended.
