You have an unknown number of red and yellow flags. But you know that there is a total of 90 flags. Since the total is fixed, there is a relationship between the number of red and yellow flags. As the number of red flags increase, the number of yellow flags decreases. This is because the total has to be fixed. You want the number of yellow flags to be at the most that it can be, which means there has to be as little red flags as possible.

With that in mind, you have the statement that in between 2 yellow flags is a number of red flags. You don't know how many, but there is at least 4 red flags. You can now imagine that the 90 flags is a repetition of one yellow and some red flags, and then repeat. Take this as one cycle, where one yellow flag is followed by some red flags. If you want the number of yellow flags to be the most, you must have the least red flags. Looking at one cycle of yellow and red flags, the least number of red flags you can have is 4, as stated by the question.

Now you have a minimum of 4 red flags and then one yellow flag per cycle. This makes a total of 5 flags on total per cycle. With 90 flags in total, you have 90/5 number of cycles. From there, you can get the number of yellow flags. Give it a try. Hope it helps