We need to assume that the acid in question is a liquid (not a solid or a gas),
and we need to assume that the 16.1% is specified as 16.1% (v/v) ,
where "v/v" means volume in volume,
meaning 16.2 milliliters of that acid are contained in 100 milliliters of the solution of acid and water.
With those assumptions, the ratio of acid volume to solution volume is
x/342 = 16.1/100 , where x= number of milliliters of acid in the 342
milliliters of solution.
x/342 = 16./100 ---> x=342x16.1/100 ---> x=55 (Rounded)