30. a) "Sampling proportion" is a slightly unusual term - in this example it refers to the proportion of the population included in the sample. So here, the systematic sampling approach selected every 100th student - this would make our Sampling Proportion 0.01, or 1% (we sampled 1% of the population). Be careful not to confuse this with "Sample Proportion", which is the proportion of our final sample who fulfil some criterion, e.g. having heard of the financial aid program (and is the statistic we use to estimate that rate in the population).
b) 1% of the total population is 0.01 x 15,000 = 150 students surveyed in total. If 90% responded, we have 90% of 150 = 135 surveys returned. This is our sample: We have a sample size of n = 135.