Plz me and explain how you got the answer plz

The decrease was 3.  (from 30 down to 27)

Always calculate the decrease as a percent of the ORIGINAL number.

        3/30  =  0.1

To change a decimal to a percent, move the point 2 places that way ==> .

                 0.1  =  10% .

When you go from 30 to 27, that's a decrease of 10% .