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Suppose that a solution $(X,Y,Z)$ of the equation $X+Y+Z=20,$with $X,Y$ and $Z$ non-negative integers,
is chosen at random.What is the probability that $X$ is divisible by $5?$
$A)\frac{1}{4}$

$B)\frac{5}{21}$

$C)\frac{2}{7}$

$D)\frac{3}{20}$
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B) 5/21?
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b.
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yes

can you explain the procedure?
+4

the procedure will be in this way:

Number of non-negative integral solutions = 22C

For finding X for divisible by 5:

X = 0 then Y + Z = 20  Number of non negative integral solutions = 21C1  = 21

X =5 then Y+ Z = 15 Number of non negative integral solutions = 16C = 16

X=10 then Y + Z = 10 Number of non negative integral solutions =11C = 11

X =15 then Y + Z = 5 Number of non negative integral solutions = 6C1  = 6

X =20  then Y + Z =0 Number of non negative integral solutions = 1C1 = 1

Sum of all these values = 21 + 16 + 11 + 6 + 1 = 55

Required probability = 55 / 22C2 = 5/21

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Thanks

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