Answer: 3 lessons
Explanation:
Matt starts swimming lessons on Day 5.
He then goes to swimming lessons every 10 days. The dates he went on lessons are therefore:
First lesson = Day 5
Second lesson = Day 5 + 10 days = Day 15
Third lesson = Day 15 + 10 days = Day 25
Fourth lesson = Day 25 + 10 days= Day 35
He therefore only went for 3 lessons because the 4th lesson was after Day 30.