He gave away 9 (20 x (3/7) =9) so 21-9=12
Answer:

And using the cdf we got:

Step-by-step explanation:
Previous concepts
The exponential distribution is "the probability distribution of the time between events in a Poisson process (a process in which events occur continuously and independently at a constant average rate). It is a particular case of the gamma distribution". The probability density function is given by:

And 0 for other case. Let X the random variable that represent the random variable of interest and we know that the distribution is given by:

We know the variance on this case given by :

So then the deviation is given by:

And if we solve for
we got:

The cumulative distribution function for the exponential distribution is given by:

Solution to the problem
And for this case we want to find this probability:

And using the cdf we got:

Answer:
-9,-4
Step-by-step explanation:
9x + 5 = 68
9x = 63
x = 7
Cole put seven books on each shelf.
Answer:
Step-by-step explanation:
2s + t = r
t = r - 2s