Isn't the first one saying 2*salary not salary^2? So it is linear. That is how I read it. Saying you have twice as much as something just means double.

You definitely wouldn't choose 4x every 4 years, because 2x every 2 years is equivalent to a payment of 4x every four years with an extra payment in between each 4 year period.

So its between 2x and 3x. This should depend on how long the stream of payments are. If it is only 6 years, then 2x is better (x +2x+4x+8x = 15x vs x + 3x +9x = 13x). However, eventually 3x every 3 years should overtake 2x every 2 years in total pay, but Im not sure the best way to figure out (w/o pen/paper/calculator) when that happens.

But if we go out another 6 years, the total payout from 2x every 2 years should be 15*8x +2x +1 = 127x , and similarly for 3x it should be 13*9x +3x +x = 121x. So for a 12 year period, 2x is still better. However by the 12th year our payout should be 64x (just for that year, not total payout) for 2x E2Y and 91X for 3x E3Y, taking it out another 6 years we see the 3x option overtakes in total pay (13*91x > 15*64x). So if the stream is for longer than 18 years 3x is better, and 2x is better for anything 13 years or less. Would have to think a bit more about length in between 13 yrs and 18 yrs.

Edit: just realized its not enough to just say 13*91x > 15*64x to conclude total pay for 3x overtakes 2x. You would have to say (13*91x + 30x) > (15*64x + 63x). since 13*91x and 15*64x represent only the total pay over the 6 year period between year 6 and 12, and doest account for the differences in earlier payments.

And theres no way I can see myself getting this far in the heat of an interview without taking an awkwardly long time. I guess thats where practicing these kinds of questions thoroughly before an interview can really pay dividends.