2HCl + Zn --> ZnCl2 + H2
adding water only changed the concentration of HCl, the number of moles didn't change so the amount of hydrogen gas would still be the same.
since the zinc was in excess anyway, adding more zinc won't change anything.
adding more HCl means you'll get more hydrogen gas.
for the sake of simplicity, since the number of mol is the same let's just consider the volume. in the initial experiment you have 100 mL of it, since you add 50 mL more, you have 150% of what you had in the first experiment. so the volume of hydrogen is 150%*240=360
if you need further explanation please quote me.