This particular homework was very intensive and important. After about 20 thumbnail you start to understand that you need to change something in your workflow to get to ~150 thumbnails before time runs out. It makes you appreciate the use of Lasso Tool vs Brush Tool. Gradients are also a big help. Generally, for very quick and rough sketches I found that it's easier to do a quick shape with a Lasso Tool and then fix it with a brush.