1. How she has done it and why he hasn't done it [is / are] mysterious.
2. Why she's got fat and others hasn't [is / are] uncertain.
3. Why men die and why people bring about war [is / are] out of my league.
Before looking down, you should check the answers on your own. (I mean, no cheating

)
It is just for more accurate answers from you.
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So, here is what I think of.
Logically thinking, "1" has a how and a why. then plural?
"2" has superficially 2 why-s, but there is just one reason. so singular?
"3" has 2 why-s, and, in fact, may have one reason for each one, which sums into two. so plural??