Well, there are 13 places that each horse can place. That means the first place has 13 options, second place has 12 options, third place has 11 options, etc. Though, we only care up to fourth place. When doing probability, we're multiplying in this case. So, here's the process:
How many ways all the horses can place = 13! = 6227020800
How many ways the first four places can place = 13*12*11*10 = 17160
Divide the second by the first and multiply by 100 for the probability as a percent:
(17160/6227020800)*100% ≈ <span>0.00027557319%
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The question is incomplete, here is the complete question:
Recall that m(t) = m.(1/2)^t/h for radioactive decay, where h is the half-life. Suppose that a 500 g sample of phosphorus-32 decays to 356 g over 7 days. Calculate the half life of the sample.
<u>Answer:</u> The half life of the sample of phosphorus-32 is 
<u>Step-by-step explanation:</u>
The equation used to calculate the half life of the sample is given as:

where,
m(t) = amount of sample after time 't' = 356 g
= initial amount of the sample = 500 g
t = time period = 7 days
h = half life of the sample = ?
Putting values in above equation, we get:

Hence, the half life of the sample of phosphorus-32 is 
Answer:
Good, this question will probably get taken down though. That happened to me.
Step-by-step explanation:
Answer:
65
Step-by-step explanation:
78-13=65 or c