Question

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Answer 1

Call the number of blue jays $b$ and the number of robins $r$

We are told there are twice as may robins as blue jays:
$r=2b$
and, there are 21 birds in total:
$r+b=21$
thus, by substitution:
$2b+b=21$
combine like terms:
$3b=21$
solve for $b$:
$b=7$
back substitute:
$r=2(7)$
$r=14$

There are 7 blue jays and 14 robins.