Call the number of robins 'r' and the number of blue jays 'b'.
We are told there are twice as many robins as blue jays, thus:
[tex]r=2b[/tex]
We are also told there are 21 birds total:
[tex]r+b=21[/tex]
By substitution:
[tex]2b+b=21[/tex]
[tex]3b=21[/tex]
[tex]b=7[/tex]
Then using the other equation with known values:
[tex]r+b=21[/tex]
[tex]r=21-b[/tex]
[tex]r=21-7[/tex]
[tex]r=14[/tex]
There are 7 blue jays and 14 robins.
Check by backsubstitution:
[tex]r+b-21[/tex]
[tex]14+7=21[/tex]
[tex]21=21[/tex] CHECKS.
