The amount of rainwater collected last year was 15.6 L, and this year it was 11.4 L, this means 15.6 L-11.4 L= 4.2 L less water was collected this year than last year.
This 4.2 L is
percent of the amount of water collected last year. Evaluating this we get:
Rounding to the nearest percent this is 27%.
Thus, the percent decrease in the amount of rainwater collected is 27%.