5*80%

This problem is probably in the fifth and sixth grades of primary school.

To find another quantity, multiply the unit 1 by the fraction of the other quantity. Another quantity here is the number of completed questions, and the unit 1 is the total number of questions given by the teacher.

the second question

2 1

2 1/ 100

Think about it, if there are four balls, 2 white and 2 black, then to ensure that you must draw black, you need to draw all white, that is, draw twice to draw all white, so that the rest are all black, and you must draw black again. So 2+ 1 time is used here.

Similarly, in this problem, there are 20 balls in each color. Draw 20 first, assuming that all 20 are of the same color, then when you draw the next one, it must be a ball of another color. Therefore, 20+ 1 time is used here.

I wish you progress in your studies.