Answer:
At the end of 7 days, it will have gained 5 minutes and 36 seconds.
Explanation:
This question can be solved using a rule of three.
It gains 8 minutes every 10 days. How much does it gains every 7 days?
7 days - x minutes
10 days - 8 minutes




Each 7 days, it gains 5.6 minutes. That is, it gains 5 full minutes, plus 0.6 = 60% of a minute, in seconds.
A minute has 60 seconds. 0.6*60 = 36
So
At the end of 7 days, it will have gained 5 minutes and 36 seconds.