Adolescence /ˌæd

It 's quite feasible (that) we'll get the money.
fruitless /ˈfruːt.ləs/ adjective
If an action or attempt to do something is fruitless, it is unsuccessful or produces nothing of value
All diplomatic attempts at a peaceful solution to the crisis have been fruitless. Opposite fruitful
futile /ˈfjuː.taɪl/ /-t ̬əl/ adjective
(of actions) having no effect or achieving nothing; unsuccessful
Attempts to get supplies to the region are futile because troops will not allow the aid convoy to enter the city. It 's quite futile try ing to reason with him - he just won't listen.
All my attempts to cheer her up proved futile.
immune /ɪˈmjuːn/ adjective
1. protected against a particular disease by particular substances in the blood
Most people who've had chicken pox once are immune to it for the rest of their lives.
He seems to be immune to colds - he just never gets them.
2. [ after verb ] not affected or upset by a particular type of behaviour or emotion
The press had criticised her so often that in the end she had become immune ( to it).
3. [ after verb ] not able to be punished or damaged by something
Journalists, he insisted, must be immune (= protected) from prosecution.
impracticable /ɪmˈpræk.tɪ.kə.bl ̩/ adjective
If a course of action, plan, etc. is impracticable, it is impossible to do in an effective way
The changes to the tax system proved impracticable as they were impossible to enforce.
improbable /ɪmˈprɒb.ə.bl ̩/ /-ˈprɑː.bə-/ adjective
not likely to happen or be true