If i had a erection and half of the tip of my penis was in her vagina whats her chance of being pregnant. not sure if there was precum. are the chances high?

No, but...: Any guy who is going to venture the head of his penis into somebody's vagina, regardless of whether it's going 1/4 of the way in, 1/2 of the way in, or all the way in, can just cover his penis with a condom. They are sold everywhere, and even given away free at many colleges and clinics. Sex creates babies... so always be ready to raised a child with the woman, if having sex with her.