(purple smooth = 75, white smooth = 28, purple wrinkled = 24, white wrinkled = 8) account for the difference between the expected and experimental values.

According to Mendel's experiment if we construct a punnet square the expected ratios should be 9:3:3:1. From our experimental results,
purple smooth = 75 ( 0.56) the expected is 0.56
white smooth   = 28 ( 0.21) expected is 0.19
purple wrinkled= 24 (0.18) expected is 0.19
white wrinkled = 8 (0.06) expected is 0.06
From this results we plug the observable and expected values in a chi-square which is used to account for the differences between the expected and observable values. Then we conclude that the deviation of the observable value from the expected value is probably due to random sampling error and therefore we accept the hypothesis, such that the results are in consistent with the law of assortment.

They both determine the exact age of rocks.