#b.# If you have a #50*lb# mass, AND #1*lb-=454*g# (I don't know, I don't use these units, especially as the States has its own special variety), then..........
#"Mass of iron in grams"-=50*cancel(lb)xx454*g*cancel(lb^-1)=??lbs#
#"Moles of iron"# #=# #(50*lbxx454*g*lb^-1)/((2xx55.85+3xx15.999)*g*mol^-1)=142*mol#.