Conditional probability of walking given long travel time

One day, at a particular school, $m$ students walked to school and the remaining $n$ students travelled to school using a different form of transport.

Of the $m$ students who walked, 20% took at least 30 minutes to get to school.

Of the $n$ students who used a different form of transport, 40% took at least 30 minutes to get to school.

Given that a randomly selected student took at least 30 minutes to get to school, the probability that they walked to school is given by