A couple of variables missing. The lappy charger is rated at 90W, but the laptop will probably not take 90W, if the internal battery is fully charged, or the laptop power settings are medium/low. Also, the 12V battery capacity is rated for a 20 hour discharge cycle(assuming lead acid or similar) Also, the inverter will obviously have losses, which may be expressed either as a % of the running load, or a fixed number of watts (or a combination of both)
At best, the Batt can supply 12 X 40 = 480W
Worst case the laptop takes a full 90W
Assume the inverter to be (say)75%efficient,
480/90 = 5.3 hrs X .75 = 4hrs
then derate the battery AH for 4 hr discharge cycle, I’d guess you’d be lucky to get 3.5 hrs (assuming the lappy takes a full 90W, which I doubt). All a bit guessworky I’m afraid.
Plus what the internal battery gives you.
HTH