Let x be the number of transactions made on ATM for each plan.
We know that their local plan charges $13 monthly and $6 for any transaction beyond six, then its cost is:
[tex]3(x-6)+13[/tex]Now, anywhere plan charges $19 monthly plus 1.75 for every transaction then its cost is:
[tex]1.75x+19[/tex]Now we want to know for how many transactions anywhere plan cost less, then we have:
[tex]\begin{gathered} 1.75x+19<3(x-6)+13 \\ 1.75x+19<3x-18+13 \\ 19+18-13<3x-1.75x \\ 24<1.25x \\ x>\frac{24}{1.25} \\ x>19.2 \end{gathered}[/tex]Therefore Anywhere plan will cost less for 20 or more transactions.
(we round up since we can't have a a decimal amount of transaction)