"Fewer cavalry officers than infantry officers become Brigadiers but a higher proportion of Brigadiers from cavalry go on to become/becomes Generals."
"A large number of civil servants is/are corrupt."
I think in the first sentence 'becomes' is correct and in the second sentence 'are' is correct just because both sound right. But can somebody please explain the rule? In the first sentence the subject is 'a higher proportion of officers' but the plural form of verb sounds right. Similarly in the second sentence, the subject is 'a large number of civil servants' but again the plural form of verb sounds right.