identify variables:
x= # of days
y= total $ amount
Equation:
121+1x=12x
subtract -1x on both sides:
121+1x=12x
-1x -1x
1x-1x cancels out and 12x-1x is 11x so new equation is:
121=11x
now divide 11 on both sides
121 divided by 11 is 11. 11 divided by 11 is 1.
x=11
Now plug x=11 in option A.
121+1(11)=y
121+11=y
132=y
Now plug x=11 to option B.
12(11)= y
132=y
After 11 days the cost of the two options will be the same and both options would cost $132.