Ateacher wants to figure out the amount of a solution that is needed for a titration experiment in a class lab. the class has 15 students, and each experiment needs 50 ml of solution.
if each student will perform the experiment two times, how many ml of solution will the teacher need for the whole class?

You need to multiply 15 by 2 since each student is doing 2 trials which gives you 30 (you can think of it as 30 students doing it once if that helps you).  Then you have to multiply 30 by 50 since the experiment is being done 30 times and each time requires 50mL which equals 1500mL.
Therefore the teacher needs to make 1500mL of solution for the lab.

I hope this helps.