The #"nickel":"zinc":"copper"# ratio of #3:4:13#
tells us that
#color(white)("XXX")#for every #3# units of nickel,
#color(white)("XXX")#there will be #4# units of zinc, and
#color(white)("XXX")#there will be #13# units of copper.
That is, out of a total of #3+4+13=20# units
#color(white)("XXX")3# will be nickel,
#color(white)("XXX")4# will be zinc, and
#color(white)("XXX")13# will be copper.
Phrased still another way:
[1]#color(white)("xxx")# #3# out of every #20# units will be nickel
#color(white)("XXX")#Out of a combined weight of #4# kgm
#color(white)("XXX")3/20xx4" kgm"=3/5" kgm"=0.6" kgm"# will be nickel.
Similarly
[2]#color(white)("xxx")# #4# out of every #20# units will be zinc
#color(white)("XXX")#Out of a combined weight of #4# kgm
#color(white)("XXX")4/20xx4" kgm"=4/5" kgm"=0.8" kgm"# will be zinc;
and
[3]#color(white)("xxx")# #13# out of every #20# units will be copper
#color(white)("XXX")#Out of a combined weight of #4# kgm
#color(white)("XXX")13/20xx4" kgm"=13/5" kgm"=2.6" kgm"# will be copper.
