And so to gets the #"volume"#..we arrange the given quotient..

#"volume of solution"="moles of solute"/"concentration"#

#a.# And so if #3.0*mol# are required from #12.0*mol*L^-1# #HCl#...we take the quotient...

#(3.0*mol)/(12.0*mol*L^-1)=0.250*L# (mind you I don't think you can get #HCl# at this concentration....)

#b.# And if #3.0*mol# are required from #2.0*mol*L^-1# #HCl#...we take the quotient...

#(3.0*mol)/(2.0*mol*L^-1)=1.50*L#

#c.# And if #3.0*mol# are required from #0.5*mol*L^-1# #HCl#...we take the quotient...

#(3.0*mol)/(0.50*mol*L^-1)=6.0*L#

#d.# And if #3.0*mol# are required from #0.010*mol*L^-1# #HCl#...we take the quotient...

#(3.0*mol)/(0.010*mol*L^-1)=300.0*L#

And clearly this last is an impratical proposition....