In this case, the unknown value is the percentage decrease. The formula to be used for calculating decrease in percentage is:
pd=((ov-dv)/ov)*100
where pd=percentage decrease, ov=original value and dv=decremented value. Substituting with the real values the calculation is:
((48-40)/48)*100=16.67%
So the percentage decrease in garbage production per week is 16.67%.