To determine the 90th percentile, you need to identify the value below which 90% of the data points fall on the graph.

To find the 90th percentile using a cumulative frequency graph, begin by determining the total number of data points. Next, multiply this total by $0.9$ to locate the position corresponding to the 90th percentile. For instance, if the total number of data points is $100$, you would calculate the position as $100 \times 0.9 = 90$. Thus, the 90th percentile corresponds to the 90th data point.

Once you have established this position, locate it on the vertical axis of the cumulative frequency graph. From this point, draw a horizontal line until it intersects the cumulative frequency curve. After identifying the point of intersection, draw a vertical line down to the horizontal axis. The value at this intersection on the horizontal axis represents the 90th percentile.

For example, if your cumulative frequency graph indicates that $90\%$ of the data points are below a value of $75$, then $75$ is the 90th percentile. This visual method enables you to clearly determine the value below which $90\%$ of the data falls, thereby providing valuable insight into the distribution of the data.