[fatimaafi45]

Given the following data:

Trial             VNaOH (mL)
  1                 17.48
  2                 17.24
  3                 17.62

a)  The volume of acetic acid used in each trial was 15.00 mL, and the density of the acetic acid solution is 1.00 g/mL.
b)  Determine which trials should be used in the analysis and which should be left out.  [runs 1 and 3 should be used]
c)  Determine the (average) concentration of the acetic acid solution.  [0.5852 M]
d)  Determine the (average) percent by mass of acetic acid in the solution. [3.51]

I understood how to do the first two questions, but the last question really has me stumped. I'm not sure if there's a formula that I need to use for the last question. I've tried searching on the internet but I found it to be unclear. This isn't really homework, this is just something for my understanding really.

[Borek]

No way to solve the problem (c, d) without knowing concentration of NaOH used.

[DrCMS]

As Borek says we can not calculate c without knowing the NaOH molarity. 
However if you have calculated the molarity of the acetic acid solution correctly as 0.5852M that is easy to convert to % acetic acid by using the molecular weight of acetic acid.