value: singular vs plural [duplicate]

Solution 1:

"Wall Street banks had made billions of dollars on complex investments backed by mortgages whose value now plunged."

In this sentence, why was the word "value" used as a singular noun here?

The singular is appropriate because each mortgage has only one value and that value has plunged on an individual basis.

"- whose value now plunged." the value of each of which has plunged.

Compare, "Children should not put their head out of the window of the train."

Solution 2:

Part of the answer is that, as you suggest, value encompasses the values of the countable mortgages. But this seems insufficient. If I write covid has decreased the values of my assets I imply that each asset (and therefore their total) has decreased. If I write covid has decreased the value of my assets I leave open the possibility that some have decreased, some may even have increased, and that the total has decreased.

I am no financial expert so I prudently assume ignorance of high finance and mortgages and adopt the singular version.