You currently have $247.
And you also take out $20, every week.
So first, work out the maximum amount of money you can take out before reaching the limit. (247 – 100 = 147).
Now that we know you can take out $147 max, you just need to divide it by $20, because that’s how much you take out every week. (147 ÷ 20 = 7.35 weeks.) But of course, the question is asking for the answer in weeks, so you round the answer down to 7 weeks.
You can last 7 weeks by taking out $20 every week from your $247 balance to avoid the low balance fee.