# Calculate ratio of liquid measurements

I have a solid laundry detergent with measurement instructions advising a ratio of 4 litres (1 gallon) hot water to 15 ml (1 tbs) of detergent.

If someone wanted a variable measurement, such as how much detergent one would use with 1 litre of water, or 500 ml of water, how would they calculate this ratio?

$4x= 15$
Where x is a litre, so if you wanted $1$ litre, $x=15/4$
if you wanted 500ml, solve for $x/2$