80% of the astronomy club members own a telescope. If 20 students own a telescope, how many students are in the astronomy club in all? help

Find x to the nearest hundredth place

How to do this , and can some one explain

List x1, x2, x3, x4 where xi is the left endpoint of the four equal intervals used to estimate the area under the curve of f(x) between x = 4 and x = 6. a 4, 4.5, 5, 5.5 b 4.5, 5, 5.5, 6 c 4.25, 4.75, 5.25, 5.75 d 4, 4.2, 5.4, 6