If we use a z-score calculator, our value of 0.9 corresponds with a z-score of 1.282. Therefore, you can either take the closest two values, 0.8997 and 0.9015, to your desired value, 0.9, which reflect the z-scores of 1.28 and 1.29, and then calculate the exact value of "z" for 0.9, or you can use a z-score calculator. This is one of the difficulties of refer to the standard normal distribution table because it cannot give every possible z-score value (that we require a quite enormous table!). There is only one problem with this z-score that is, it is based on a value of 0.8997 rather than the 0.9 value we are interested in. This forms the second part of the z-score. This time, the value on the x-axis for 0.8997 is 0.08. We now need to do the same for the x-axis, using the 0.8997 value as our starting point and following the column up. You will notice that the value on the y-axis for 0.8997 is 1.2. If we take the 0.8997 value as our starting point and then follow this row across to the left, we are presented with the first part of the z-score. When looking at the table, you may notice that the closest value to 0.9 is 0.8997. As such, we first need to find the value 0.9 in standard normal distribution table. We know the percentage we are trying to find, the top 10% of students, corresponds to 0.9.
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