The following wind data $($direction and speed$)$ were received from $50$ km to the east, north, west, and south of a station, respectively: $90◦,10$ms$-1$; $120◦,4$ms$-1$; $90◦,8$ms$-1$; $60◦,4$ms$-1.$

$($a$)$ Calculate the approximate horizontal divergence at the station.

$($b$)$ Suppose the given wind speeds were all in error by $$pm $10\%.$ What would be the percentage error in the calculated horizontal divergence in the worst case? 

$($c$)$ Does the answer in $($b$)$ suggest why horizontal divergence is not usually a measured quantity? What is the reasoning behind your answer?