To answer this question, we have to multiply the volume that each student needs times the number of students:
[tex]40\cdot120mL=4800mL[/tex]Convert this volume to liters (1L=1000mL):
[tex]4800mL\cdot\frac{1L}{1000mL}=4.8L\approx5L[/tex]It means that the assistant must order 5L of acid.