a) The graph needs to be at least 7 squares wide, because if each square goes up by 5, the biggest number it will need to fit is 35. So you need to add a square to again and again until you get to 35.
b) The biggest number here is 1.9, if we want the best resolution. we should go up in 0.5s, it may not fill in all the squares but it will include 1.9, however a more specific answer can be found by doing 1.9 ÷ 20 = 0.095