Help please I might fail…

Multiplying an even number of negative numbers gives an answer that is choose from:

Aclass consists of 55% boys and 45% girls. it is observed that 25% of the class are boys and scored an a on the test, and 35% of the class are girls and scored an a on the test. if a student is chosen at random and is found to be a girl, the probability that the student scored an ais

Aclassmate thinks that solving a system by graphing gives an exact answer when the lines appear to cross at a grid point, but only an approximate answer when they don't. explain why this isn't true.