A maths lecturer called Dr. X, is occasionally late for lectures. Dr. X has two units, 1030 and 2051 and he is late 10% of the time for 1030 and 15% of the time for 2051. You overhear two students remaking that the good Dr was late again. What is the probability that this remark came from an 1030 student? You can assume that 75% of Dr. X's students are in 1030, 25% are in 2051 and that no student takes both units.
Here is what I did but I got a probability of 3.19 which is definitely wrong. Can someone find out what I did wrong? As I can't find it, thanks