Question

A television advertising capaign is conducted during the fooall season to promote a well-known brand X shaving cream. For each of several weeks, a survey is made, and it is found that each week, 80% of those using brand X continue to use it and 20% switch to another brand. t is also found that of those not using brand X, 20% swich tobrand X while the other 80% continue using another brand.

If 20% of the people are uing brand X at the start of the dvertiing campaign, what percentage will be using it 1 week later? 2 weeks later?