If the first week it increased by 20%, then an easy way to write this as an equation is 20 (initial number of views) x 1.2 (because it increased by 20%).
If it increased by 20% for 4 weeks, then you raise 1.2 to the power of 4.
20 x 1.2^4 = 41.472
Rounded to the nearest whole number this would be 41.
You can check this because 20% of 20 is 4. So the first week would have 24 views.
Second week, 28.8 views (20% of 24 is 4.8)
Third week, 34.56 views (20% of 28.8 is 5.76)
Fourth week, 41.472 views (20% of 24.56 is 6.912)
Of course, it's easier to just calculate is as 20 x 1.2^4