Maths probability question help

On any school day, the probability that Josh oversleeps is 1/5. If he oversleeps, the probability that he will remember all his books is 2/9. If he does not oversleep, the probability that he will remember all his books is 5/7. Use a tree diagram to work out the probability that Josh will not remember all his books tommorow.

Reply 1

1jonam16

(1/5*7/9) + ( 4/5 *2/7)

Reply 2

f_und21

okay you need to draw out a tree diagram so i would draw one out with both routes because it won't let me draw one. You need to have two columns with the first one being oversleeps then yes/no, then the second being remember books yes/no for both of the first outcomes (so you will end up with a yes/no then from the yes a yes/no and from the no a yes/no

Once you have done this, you need to follow both the routes (Yes Yes) and (No Yes) and you have to add them because you're working out an 'or' so the probability of either occouring. So, it could be Yes Yes (1/5 * 2/9 = 2/45) or No Yes (4/5 * 5/7)= 20/35 (this cancels to 4/7, which is equivalent to 36/45)

You then add the two probabilities together to make 38/45.

hope this helped

(edited 8 years ago)

Reply 3