Calculate the volume (in ml) of a 2.75 m solution that must be used to make 1.25 l of a 0.150 m solution.

This is a dilution question where a certain volume of a concentrated solution is taken and a diluted solution is prepared.
the following equation is used,
C1V1 = C2V2
C1 is concentration and V1 is volume of concentrated solution 
C2 is concentration and V2 is volume of diluted solution prepared
substituting these values in the equation,
2.75 M x V = 0.150 M x 1.25 L
V = 0.0681 L
a volume of 68.1 mL should be taken from the 2.75 M solution to prepare the 0.150 M solution

The answer to that would be magnesium