67.0 mL of a 1.50 M solution is diluted to a volume of 258 mL. A 129-mL portion of that solution is diluted using 163 mL of water. What is the final concentration?

Answer 1

Assuming that the 129 mL plus the 163 mL added water adds to a total volume of 129+163. (That's the same thing as saying that I'm assuming the volumes are additive.
1.50 x (67.0 mL/258 mL) x [129 mL/(129+163)]