A student has to be in one of those four ranks. If a student is not a freshman or sophomore, he must be a junior or a senior. All students make up 100% of the total number of students. Since the freshmen are 35% of the students, and the sophomores are 30% of the students, then they make up 65% of all students. 100% - 65% = 35%. That means the juniors and seniors combined account for 35% of all students. A student selected randomly has a 35% probability of being a junior or a senior.
<span>log (M/N) = log M - log N so log M - log N = 5
log (P/N) = log P - log N so log P - log N = 7
If you subtract equation 2 from equation 1, you have log M - log P = -2
That means that log (M/P) = -2 ----> 10^-2 = M/P ---> P = 100M</span>