The number of moles of hydrogen produced will mirror the number of moles of #Zn# that react, or, from a different perspective, will be half the number of moles of #HCl# that react.
The balanced chemical equation for this reaction is
#Zn_((s)) + 2HCl_((aq)) -> ZnCl_(2(aq)) + H_(2(g))#
Now look at the mole ratios you have between #Zn#, #HCl#, and #H_2#: 1 mole of #Zn# needs 2 moles of #HCl# to produce 1 mole of #H_2#.
All you have to do now is figure out exactly how many moles you have. This is done by using the molarity of the #HCl# solution to calculate how many moles of #HCl# you have, since you know that #Zn# is in excess.
#C = n/V => n_("HCl") = C * V = "0.40 M" * 400*10^(-3)"L" = "0.160 moles"#
Therefore, the number of #H_2# moles will be
#"0.160 moles HCl" * ("1 mole H"_2)/("2 moles HCl") = "0.800 moles"# #H_2#
