Explanation:
I'm going to assume 50.25 was a typo error and it is $0.25 for each extra minute.
For a 10 minute phone call, the first 5 minutes cost $3 and the other 5 minutes cost $0.25 * 5 = $1.25. Altogether it costs $4.25.
For a 15.5 minute phone call, the first 5 minutes cost $3 and the other 10.5 minutes cost $0.25 * 11 = $2.75. Altogether it costs $5.75.