200 sophomores will attend the picnic.
Since each of them will pay $0.75, the amount gathered by the 200 sophomores is:

They have $150 and they need $250 for the permit to use the city park.
Thus, the amount they need to gather from the guest is:

To pay for the permit, the amount from the guest has to be equal to or greater than 100.
This inequality can be represented as 0.75x, where x is the number of guests, and that has to be greater than 100 because 0.75x is the amount they will gather from the guests.
The inequality is as follows:

And now we solve this inequality for x by dividing both sides by 0.75:

133.33 guests must attend. Since we are talking about people, we need to round this number up to 134 people.
Answer:
134 guests