Call the number of blue jays [tex]b[/tex] and the number of robins [tex]r[/tex]
We are told there are twice as may robins as blue jays:
[tex]r=2b[/tex]
and, there are 21 birds in total:
[tex]r+b=21[/tex]
thus, by substitution:
[tex]2b+b=21[/tex]
combine like terms:
[tex]3b=21[/tex]
solve for [tex]b[/tex]:
[tex]b=7[/tex]
back substitute:
[tex]r=2(7)[/tex]
[tex]r=14[/tex]
There are 7 blue jays and 14 robins.
