In a city school, 70% of students have math class, 65% have science class, and 45% have math and science class this semester. what is the probability (rounded to the nearest whole percent) that a randomly selected student will have science class this semester, given that the student is in math class?

P(a given b) = p(a and b) / p(b)  p(a and b) is the probability student takes both a math class and a science class.  70% take math, then p(a) = .70if 65% take science, then p(b) = .6545% take both, then p(a and b) = .45  p(a given b)  is now .45 / .70 = .6428571429.  that's the probability that a randomly selected student will take science given that he takes math.  assume 45 students take math and science.assume 25 students take math only.assume 20 students take science only.  the total number of students is 90 that take either math or science.  out of that 90, 70 take either math or math and science.that's 25 for math only and 45 that take math and science.so, out of the 70 who take math, 45 also take science.that's a percentage of 45/70 = .6428571429 your answer *64.29%*