Re: having a goal date is good for some
i say keep the bet -- just extend it or change it a bit. set yourself up for success -- that's the best approach. just tell your mom that you want to succeed, did some research and here's what you propose. it's still a worthwhile bet (challenge), since weight loss is never e-a-s-y.
losing 60
pounds in 3 weeks is probably unrealistic, but losing 60 in 3 months -- i bet you could do that. or, losing 25-30
pounds in 3 weeks on water is probably doable.
contrary to other opinions, i think that ANY reason to get healthy is valid! i also think that a fast is a fabulous way to turn the 'switch' from unhealthy to healthy.
i've done several successful fasts. maintaining the loss is trickier. you can do it though, just get informed! use the non-eating time to educate yourself. knowledge is power.
to give you some numbers -- i most people lose 1/2 - 1 pound a day on fasts (higher loss rate in the beginning, then slows down).
good luck -- hope you win!