For five individuals the blood platelet aggregation was measured before (x1) and after (y1) they performed a certain task. these five pairs of values gave the following results: ¯x1 = 30.2, y¯1 = 39.0, cov(x1, y1) = 88.5. for another six individuals the blood platelet aggregation was also measured before (x2) and after (y2) they performed the same task. these six pairs of values gave the following results: ¯x2 = 52.17, y¯2 = 63.67, cov(x2, y2) = 148.67. find the covariance of the combined 11 pairs.