Final answer:
Myrna's next goldfish feeding day after Thursday is Sunday. Continuing this pattern, the 15th feeding will also fall on a Sunday.
Step-by-step explanation:
If Myrna fed her goldfish on Thursday and will feed it every third day, we need to determine the sequence of feeding days. We can start by counting the days from Thursday: the first day after Thursday is Friday (day 1), then Saturday (day 2), and the third day is Sunday. Therefore, Myrna will feed her goldfish next on Sunday.
To find out on what day of the week the 15th feeding will occur, we count in multiples of three starting from Thursday (day 0). Each cycle of seven days will bring us back to a Thursday, since 7 is a multiple of the concerned days. Therefore, 14 feeding days will bring us to two cycles of 7 days (14 is a multiple of 7) and back to Thursday. The 15th feeding will be three days after that Thursday, which is a Sunday. Therefore, the 15th feeding will happen on a Sunday.