SOLUTION: Two students are registered for the same class and attend independently of each other, student A 70% of the time and student B 60% of the time. The teacher remembers that on a give

This is conditional probability.  The probability that A was in class, given
that A or B was in class that day.

The formula is

          P(X and Y) 
P(X|Y) = ------------
            P(Y)

where X = A,  Y = (A or B)


              P(A and (A or B))     P(A)
P(A|A or B) = ----------------- = ---------
                  P(A or B)       P(A or B)

We know P(A) = .70.  We need P(A or B) 

The complement of 'at least 1' is 'none at all'.

student A attends 70% of the time.
Therefore A is absent 30% of the time.  Probability .3
Student B attends 60% of the time. Probability .6 
Therefore B is absent 40% of the time. Probability .4

They are both absent (.3)(.6) = .18 or 18% of the time.
So 1 or the other is present 82% of the time.  Probability =.82.

              P(A and (A or B))     P(A)        .3      30     15
P(A|A or B) = ----------------- = --------- = ------ = ---- = ----
                  P(A or B)       P(A or B)    .82      82     41

Probability about .366 or 36.6%of the time.
 
Edwin