Final answer:
The third group should clean 0.35 (or 35%) of the park.
Step-by-step explanation:
To find out how much the third group should clean, we need to subtract the amount cleaned by the first two groups from the total. The first group cleaned 2/5 (or 0.4) of the park, and the second group cleaned 1/4 (or 0.25) of the park. Therefore, the total amount cleaned by the first two groups is 0.4 + 0.25 = 0.65. To find out how much the third group should clean, we subtract 0.65 from 1 (since 1 represents the whole park). So, the third group should clean 1 - 0.65 = 0.35 (or 35%) of the park.