read carefully and try to understand the followings (not question asked)
water hyacinths
water hyacinths grow rapidly-in about one week we can expect the area covered bu plant's leaves to double. here's the question : if only 1% of a lake is covered in water hyacinth, how long will it be before if it's 10% covered? we may easily see that it will take a bit over three weeks, because if it's 1% covered now , next week(week 1) it is 2% ; the week after (week 2) , 4% ; the week after that (week 3), 8%. not too bad. so, we think (thinking linearly) that it will take 10 time as long to cover the lake-over half a year . Because the growth is exponential, however, it actually will take only a little over 13 weeks, about one-third what we might have expected
it takes less than five weeks for the 1% cover to become 20% cover. and it takes less than three weeks more to completely cover the lake. by the time the lake is half covered, five and two-thirds weeks after it was 1% covered, it will take only 1 more week to completely cover the lake.