A mango farm has 500 trees. Twenty trees were picked at random and their mean yield was 84 mangoes per tree. Using this sample, estimate the total yield of the whole farm.

A sample mean is used as an estimate of the mean for the whole population, so each of the 500 trees is taken to yield about 84 mangoes and the farm's total is estimated at $84\times 500=42,000$ mangoes. The value $1,680=84\times 20$ is the total for the twenty sampled trees only, which is what the sample actually produced rather than what the farm produces. The value 2,100 divides 42,000 by the sample size a second time, applying the 20 twice over. The value $8,400=84\times 100$ is a place-value slip. Note that this is an ESTIMATE: the sample was random, so it should be close, but a different twenty trees would give a somewhat different figure, and the larger the sample the steadier the estimate becomes.