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Permutations and Combinations question

2024 · 5 Apr · Shift 1 · Q54
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Permutations and Combinations question

2024 · 5 Apr · Shift 1 · Q54

JEE MainMathematicsPermutations and CombinationsNumerical+4 / −1
The number of ways of getting a sum 16 on throwing a dice four times is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 125

We need the number of ordered outcomes (x1,x2,x3,x4)(x_1,x_2,x_3,x_4)(x1​,x2​,x3​,x4​) when a die is thrown 4 times such that x1+x2+x3+x4=16,x_1+x_2+x_3+x_4=16,x1​+x2​+x3​+x4​=16, where each xi∈{1,2,3,4,5,6}x_i\in\{1,2,3,4,5,6\}xi​∈{1,2,3,4,5,6}.

Step 1: Convert to a nonnegative integer equation

Let yi=xi−1(i=1,2,3,4).y_i=x_i-1 \quad (i=1,2,3,4).yi​=xi​−1(i=1,2,3,4). Then each yi≥0y_i\ge 0yi​≥0 and yi≤5y_i\le 5yi​≤5, and y1+y2+y3+y4=16−4=12.y_1+y_2+y_3+y_4=16-4=12.y1​+y2​+y3​+y4​=16−4=12.

So we need the number of nonnegative integer solutions of y1+y2+y3+y4=12,y_1+y_2+y_3+y_4=12,y1​+y2​+y3​+y4​=12, with the restriction yi≤5y_i\le 5yi​≤5.

Step 2: Count all nonnegative solutions without the upper bound

Ignoring the condition yi≤5y_i\le 5yi​≤5, the number of nonnegative solutions is (12+4−14−1)=(153)=455.\binom{12+4-1}{4-1}=\binom{15}{3}=455.(4−112+4−1​)=(315​)=455.

Step 3: Subtract solutions where at least one variable exceeds 5

We now subtract cases where some yi≥6y_i\ge 6yi​≥6.

For a particular variable, say y1≥6y_1\ge 6y1​≥6, write y1′=y1−6≥0.y_1'=y_1-6\ge 0.y1′​=y1​−6≥0. Then y1′+y2+y3+y4=12−6=6.y_1'+y_2+y_3+y_4=12-6=6.y1′​+y2​+y3​+y4​=12−6=6. Number of solutions: (6+4−13)=(93)=84.\binom{6+4-1}{3}=\binom{9}{3}=84.(36+4−1​)=(39​)=84.

There are 4 choices for which variable is at least 6, so total to subtract initially: 4⋅84=336.4\cdot 84=336.4⋅84=336.

Step 4: Add back intersections

If two variables are both at least 6, say y1,y2≥6y_1,y_2\ge 6y1​,y2​≥6, then let y1′=y1−6,y2′=y2−6.y_1'=y_1-6,\quad y_2'=y_2-6.y1′​=y1​−6,y2′​=y2​−6. This gives y1′+y2′+y3+y4=12−12=0.y_1'+y_2'+y_3+y_4=12-12=0.y1′​+y2′​+y3​+y4​=12−12=0. So there is exactly (33)=1\binom{3}{3}=1(33​)=1 solution for each such pair.

Number of pairs: (42)=6.\binom{4}{2}=6.(24​)=6. So we add back 6⋅1=6.6\cdot 1=6.6⋅1=6.

Step 5: Higher intersections

Three variables cannot each be at least 6, because that would require sum at least 18>1218>1218>12. So no further terms occur.

Step 6: Final count

By inclusion-exclusion, 455−336+6=125.455-336+6=125.455−336+6=125.

Therefore, the number of ways is 125.\boxed{125}.125​.

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