Tree Diagrams A* GCSE

Please help, I can't do this question and I'm fretting horribly. I need someone to explain it to me.
"Tom and Sam have been practising percentage change questions for their maths exam.The probability that Tom gets a question right is 0.8. The probability that Sam gets a question right is 0.6. They both attempt another question. Work out the probability that at least one of them gets it correct".
Tree diagrams are my worst, I need serious help. Thank you.

Reply 1

Krollo

The probability of at least one getting it right is the probability of them both not getting it wrong.

If the probability of Tom getting it right is 0.8, then the probability if him getting it wrong is 1 - 0.8 = 0.2.

Can you work from there?

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Reply 2

TheSweatyNerd

0.8 * 0.4 = 0.32
0.6 * 0.2 = 0.12
0.32 + 0.12 = 0.44
Brav, this is basic mathematics.

Reply 3

Caseyleia

Original post by TheSweatyNerd

0.8 * 0.4 = 0.32
0.6 * 0.2 = 0.12
0.32 + 0.12 = 0.44
Brav, this is basic mathematics.

The answer my book gives is 0.92

(edited 9 years ago)

Reply 4

Caseyleia

Original post by Krollo

I did this and I have the tree diagram drawn out, but I cannot get the answer my answer book is saying :frown:

Reply 5

Krollo

Original post by Caseyleia

I did this and I have the tree diagram drawn out, but I cannot get the answer my answer book is saying :frown:

OK. The probability of Tom getting it wrong is 0.2, and the probability of Sam getting it wrong is 0.4. If they both get it wrong, we multiply the probabilities to get 0.08. Can you work from there?

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(edited 9 years ago)

Reply 6

Connorbwfc

Original post by TheSweatyNerd

0.8 * 0.4 = 0.32
0.6 * 0.2 = 0.12
0.32 + 0.12 = 0.44
Brav, this is basic mathematics.

You got the wrong answer.

The probability Tom doesn't get it right is 0.2, the probability Sam doesn't get it right is 0.4. For there to be less than one they would both have to get it wrong. The probability of this is 0.4*0.2=0.08. Therefore the probability it is 1 or more is 1-0.08=0.92.

Reply 7

interstitial

Original post by TheSweatyNerd

0.8 * 0.4 = 0.32
0.6 * 0.2 = 0.12
0.32 + 0.12 = 0.44
Brav, this is basic mathematics.

Clearly not, because your answer is incorrect.

caseyleia

X

Start by thinking of the probabilities of them getting it wrong. If the probability of A getting it right is 0.8, and the sum of the probabilities is 1, what is the chance of him getting it wrong.

I've enclosed my solution in the spoiler below, but only click on it if you're really stuck.

The only probability that leads to no one getting a question right is if A and B get it wrong. The probability of A getting it wrong is 0.2 and probability of B getting it wrong is 0.4. The likelihood that these will happen together is the product of the two which is 0.08.

As the sum of the probabilities is 1 (as at least one of them will happen), to find the probability of the rest you can simply do 1-0.08=0.92.

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(edited 9 years ago)

Reply 8

Caseyleia

Original post by Connorbwfc

Thank you!!!! I felt stupid before but I completely understand now :smile:

Reply 9

Caseyleia

Original post by Arithmeticae

Clearly not, because your answer is incorrect.

Start by thinking of the probabilities of them getting it wrong. If the probability of A getting it right is 0.8, and the sum of the probabilities is 1, what is the chance of him getting it wrong.

I've enclosed my solution in the spoiler below, but only click on it if you're really stuck.

Working

Posted from TSR Mobile

Thank you very much!!!!