To decrease an amount by 85% what single multiplier would i need to use?

You are asking about what number must you multiply another random number by to make that random number decrease by 85% of that random number, right?

If the above is correct, then the number is 0.15

because 100% - 85% = 15% and 15% = 15 / 100

= 0.15

The correct answer will be

1.85