First you want to find out how much money Anna has left after the first five minutes of her phone call. You find this value by the subtracting the cost of the first five minutes by the original amount of money that Anna has.
1.35-0.60 = 0.75
This means after the first five minutes, Anna only has 0.75 cent left to spend. In order to find out how many additional minutes she talked you divide her remaining change by the cost of each additional minute.
0.15x =0 .75
This means Anna can talk an additional 5 minutes before she runs out of money. To find out how long she can talk, you add the additional minutes she can talk after the first five minutes to the five minute.
5 + 5 =10
Anna can talk for 10 minutes before she runs out of money.