Rafael is taking a test that contains a section of 12 true-false questions. how many of the possible groups of answers to these questions have exactly 8 correct answers of false? a. 425 c. 500 b. 495 d. 515

495

We want exactly 8 correct answers of false, so we want exactly 4 correct answers of true.  In short, we want something like . Because the sequence of T values is shorter than the F values, this means it's a lot easier to count out the number of ways to pick the true responses, rather than the false responses.

Consider we have 4 slots: A, B, C, D

In slot A, we can pick the number 1 through 12 to represent getting a T

For slot B, we have 11 choices because we can't reuse whatever we picked for slot A

Then for slot C we have 10 choices

Finally, slot D has 9 choices

Multiply those values out: 12*11*10*9 = 11,880

If order mattered, then we'd be done with the problem. However, order does not matter. This is because a sequence like {1, 2, 3, 4} is the same as {1,2,4,3}. The only thing that matters is that we have the numbers in the same group. We can rearrange any given group however we want.

Note that there are 4! = 4*3*2*1 = 24 ways to arrange any individual group of four items. So we have to divide the previous result 11,880 by 24 and we get: 11880/24 = 495 which is the answer

If you wanted, you can use the nCr combination formula to get this same answer. You would use n = 12 and r = 4

54 kilometers per hour

step-by-step explanation:

according to rules:

1 km = 1000 meter

which means that 1 meter = 1/1000 km

1 hour = 3600 second [ as 1 hour = 60 min = 60 *60 seconds ]

which means that 1 second = 1/3600 hour

therefore, 1 kilometer/hour =

thus to convert 15 meter per second to kilometer per hour, we multiply 15 to 3.6 kilometer/hour.

15 meter/second=   which is the required answer.

ummm seems that u didnt post the attachement !