You can track the number though it would takes time.
N = Negin and W = Weaver
Day zero:
N = 5
W = 21
Day one:
N = 8
W = 20
Day two:
N = 11
W = 19
Day three:
N = 14
W = 18
Day four:
N = 17
W = 17
So it takes them 4 days to have the same number of boxed of cookies (17 boxes)
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Now for algebra part:
d stands for days
Equation for Negin would be N = 5 + 3d
(because for each day, Ms.Negin would get 3 more boxes)
and Weaver would be W = 21 - d
(because for each day, Mrs.Weaver would eat 1 box)
So to solve it algebraically, set N and W equals to each other, like this:
5+3d = 21 - d
Now solve for d:
add both sides by d:
5 + 4d = 21
subtract both sides by 5:
4d = 16
Now divide both sides by 4:
d = 4
so 4 days.
To find the number of boxes on 4th day, you can just plug in one equation: Let's go with Weaver's:
W = 21 - d
W = 21 - 4
W = 17
so 17 boxes.
Hope this helps!