Question

Jones like to watch the bird in her yard one morning she notices that there were twice as many robins as Blue Jays. if she counted 21 Blue Jays and Robbins in all how many blue jays did she count?

Answer 1

R=number of robbins
b=number of blue jays


twice as many robins as blue jays
r=2b


21 blue jays and robbins
r+b=21
subsitute
r=2b so subsitute 2b for r
2b+b=21
3b=21
divide both sides by 3
b=7

she counted 7 blue jays

Answer 2

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:

$r=2b$

We are also told there are 21 birds total:
$r+b=21$

By substitution:
$2b+b=21$
$3b=21$
$b=7$

Then using the other equation with known values:
$r+b=21$
$r=21-b$
$r=21-7$
$r=14$

There are 7 blue jays and 14 robins. 

Check by backsubstitution:
$r+b-21$
$14+7=21$
$21=21$ CHECKS.