Question

To decrease an amount by 18%, what single multiplier would you use?

Answer 1

If a number decreases by 18% then it is down to 82 % of its value, so your multiplier is 88% written as a decimal number -- 82/100 = 0.82 - not 0.88

Answer 2

original number is 100%
100% decreased by 18% means
100% -18%=82%

multiplier is 82% aka 0.82