If 3.5 ml of distilled water is added to 1.5 ml of a 0.150M solution of CoCl2 what is the concentration of the resulting solution? please show the work

No one should ask a question like this. To give an answer we must ASSUME that the volumes are additive and most times they are not.

Why don't you reason this out.
You have a 0.150 M soln. It has been diluted from 1.5 mL to 5.0 mL so shouldn't the new concn be
0.15M x (1.5 mL/5.0 mL) = ?M assuming the volumes are additive and the total volume is 5.0 mL.