first equation:
M=152+7x
where m=monthly and x is per minute of use
second equation:
M=95+9x
where x is per minute use of phone and m=month.
Just substitute:
M=M
152+7x=95+9x
57=2x
x=28.5 or 28 minutes 30 seconds.
Substitute:
M=152+7(28.5)=351.5
M=95+9(28.5)=351.5
Therefore you need to use the phone for 28 minutes and 30 seconds to have the same amount of cost for plan 1 and 2. Hope this helps =)
