To calculate 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.
Each student talks for 20 minutes, and there are 30 students:
\[ 30 , \text{students} \times 20 , \text{minutes/student} = 600 , \text{minutes} \]
Therefore, the total time needed to present all of the science projects is 600 minutes.