Step-by-step explanation:
When working with dillutions we usually use this formula.
Vd * Md = Vc * Mc
Where Vc and Mc are the volume and molarity of the concentrated solution and Vd and Md are the volume and molarity of the dilluted solution.
Vd = ? Md = 2 M Vc = 20 mL Mc = 7.5 M
With that formula we can find the volume of the dilluted solution that we can prepare from a determined volume of a concentrated solution.
Vd * Md = Vc * Mc
Vd = Vc * Mc/Md
Vd = 20 mL * 7.5 M/(2 M)
Vd = 75 mL
With the concentrated solution we can prepare 75 mL of the 2 M solution. To prepare that solution we will add some mL of water to the concentrated solution that already had a volume of 20 mL.
Vdilluted = Vwater + Vconcentrated
Vwater = Vd - Vc
Vwater = 75 mL - 20 mL
Vwater = 55 mL
Answer: we have to add 55 mL of water to the 7.5 M solution.