To find out the total time needed for all 30 students to present their science projects, you can multiply the number of students by the time each student gets to talk.
There are 30 students, and each student presents for 20 minutes:
\[ 30 \text{ students} \times 20 \text{ minutes/student} = 600 \text{ minutes} \]
So, the total time needed to present all of the science projects is 600 minutes.