A survey found that 7 out of 30 students walk to school.?

First count the total ways you could get the desired outcome. From the 7 walkers, you want to pick 4 for the survey:
C(7,4) = (7 x 6 x 5 x 4) / (4 x 3 x 2 x 1) = 35 ways

Then count the ways, from the 30 students to pick *any* 4 for the survey:
C(30,4) = (30 x 29 x 28 x 27) / (4 x 3 x 2 x 1) = 27,405 ways

Divide to get the probability:
35/27405

But you can reduce that by dividing top and bottom by 35.

Answer:
P(all 4 walk to school) = 1/783

Hint. The answer involves 7C4 and 30C4