Date | May 2018 | Marks available | 2 | Reference code | 18M.1.SL.TZ2.S_8 |
Level | Standard Level | Paper | Paper 1 | Time zone | Time zone 2 |
Command term | Find | Question number | S_8 | Adapted from | N/A |
Question
Pablo drives to work. The probability that he leaves home before 07:00 is 3434.
If he leaves home before 07:00 the probability he will be late for work is 1818.
If he leaves home at 07:00 or later the probability he will be late for work is 5858.
Copy and complete the following tree diagram.
Find the probability that Pablo leaves home before 07:00 and is late for work.
Find the probability that Pablo is late for work.
Given that Pablo is late for work, find the probability that he left home before 07:00.
Two days next week Pablo will drive to work. Find the probability that he will be late at least once.
Markscheme
* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.
A1A1A1 N3
Note: Award A1 for each bold fraction.
[3 marks]
multiplying along correct branches (A1)
eg 34×1834×18
P(leaves before 07:00 ∩ late) = 332332 A1 N2
[2 marks]
multiplying along other “late” branch (M1)
eg 14×5814×58
adding probabilities of two mutually exclusive late paths (A1)
eg (34×18)+(14×58),332+532(34×18)+(14×58),332+532
P(L)=832(=14)P(L)=832(=14) A1 N2
[3 marks]
recognizing conditional probability (seen anywhere) (M1)
eg P(A|B),P(before 7|late)P(A|B),P(before 7|late)
correct substitution of their values into formula (A1)
eg 3321433214
P(left before 07:00|late)=38P(left before 07:00|late)=38 A1 N2
[3 marks]
valid approach (M1)
eg 1 − P(not late twice), P(late once) + P(late twice)
correct working (A1)
eg 1−(34×34),2×14×34+14×141−(34×34),2×14×34+14×14
716716 A1 N2
[3 marks]