A substance goes into a solution at the rate of 25% of the undissolved portion each minute. If the initial amount was 64/27 grams, what amount (in grams) is undissolved after 6 minutes?

A substance goes into a solution at the rate of 25% of the undissolved portion each minute. If the initial amount was 64/27 grams, what amount (in grams) is undissolved after 6 minutes?