where k is the rate, and y is the proportion of the moisture.

so,

Integrate both sides:

You will get:

..................(1)

Use this equation to find out what your question is asking. First start with time to find out . At time , the towel

has not lost any moisture. That means

So,

Now find k. To find k, use the first part of your question.

At time , the moisture lost is 10%. So the remaining moisture in the towel is .

So,

Now, that you have and , find .

Let be the time when 70% moisture is lost, that is, the remaining moisture is 30. So, use equation (1) in a similar manner to find the time(t).