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:
Now, anywhere plan charges $19 monthly plus 1.75 for every transaction then its cost is:
Now we want to know for how many transactions anywhere plan cost less, then we have:
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)