Expected Trials in Random Sampling

To select a random sample of 10 students from a class of 80, a teacher uses a random digit table. The students are numbered from 01 to 80. Each two-digit number read from the table has a $0.80$ probability of being valid (i.e., in the range 01–80). What is the expected number of two-digit numbers the teacher must read to identify 10 distinct students for the sample?