To do a motsi for bread we need to eat 56 gr.. How if I eat half bread and half mezonoth? or less than 56 gr. for both of them.

Lechatchila one should eat 56 grams of bread in a meal. However, if he only ate a Kezayit (aprox. 30 grams) he still says Birkat Hamazon (Grace after meal). When saying Birkat Hamazon he is blessing also the Mezonot he ate. Rabbi Yitzchak Grinblat