Consider the procedure described in "Adaptive Playout Delay" for estimating average delay di. Let r1 - t1 be the most recent sample delay, let r2 - t2 be the next most recent sample delay, and so on. For a given audio application, suppose four packets have arrived at the receiver with sample delays r4 - t4, r3 - t3, r2 - t2, and r1 - t1. Express the estimate of delay d in terms of u and the four samples.