Hi Shaun,
Under the regulations, variable fuel is only required from origin to destination (but not to the alternate) - and that too for charter and not necessarily private operations.
Fixed reserves are 15 gallons for all operations (per the company policy in the Echo supplement)
In this question, you arrive overhead the destination with 50 gal on-board (we don't really know if variable was used or even required for this flight - it doesn't matter). However we assume the fixed reserve is intact (since the question doesn't comment on it).
So 50 gallons overhead the destination, less 15 gal (fixed reserves) = 35 gal remaining for safe endurance purposes overhead the destination.
Of the 35 gal, 26 gal is required to get you to the alternate aerodrome (remember alternate doesn't require variable), leaving 9 gal of margin that you can use for holding over the destination (in the hope the wheels up is cleared away in that time
)
9 gal at 17.6 gph (35% MCP for holding per the echo manual) = 31 minutes of holding over the destination before you must divert.
If you arrived at the destination at 0412 UTC, you can hold for 31 min and at 0443 UTC you must begin your diversion to the alternate, where all going well you arrive with exactly 15 gallons of fixed reserves remaining.
