There can be no negative in a sqrt
so x+1 must be equal or greater then 0
x+1

0
x

-1
so all answers must be greater then or equal to -1
-1,0,1,2,3,4,5,6,7,8
(-1,infinity) is the domain
Since the question only asks for bagels, put the cream-cheese idea aside.
The total spent has to be less than $7 so an equation can be this: $0.75x<$7 where x is the number of bagels
Let be be Beth, bi be Bill, and e be edie. Then:
1/be + 1/bi +1/e=1/12
1/bi + 1/e=1/15
10/be +10/bi +10/e+5/be+5/bi=1
Solve for be, bi, and e
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