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After reading your post, I have a question.
It is about this passage "The sum of their ages WAS 36."
To which time moment this statement does relate ?
To the past ? (6 years ago).
Or to the present ? (now).
If it relates to the present, then why you use "WAS" ?
If to the past, then it should sound in other way :
"The sum of their ages WAS 36 years then".
Honestly, I do not need your clarification.
===--->>> It is YOU, "the composer", who needs it. <<<---===