Order in the court ....

How many ordered quadruples ( a , b , c , d ) (a,b,c,d) , where a , b , c , d a,b,c,d are non-negative integers and a b c d a \le b \le c \le d , satisfy the equation a + b + c + d = 15 a + b + c + d = 15 ?


The answer is 54.

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2 solutions

Ishan Kothari
Oct 31, 2014

Same as of Brian Charlesworth

There probably is a more elegant way to do this, by for what it's worth, here is my case by case approach.

(i) If ( a , b ) = ( 0 , 0 ) (a,b) = (0,0) then ( c , d ) (c,d) can be ( 0 , 15 ) , ( 1 , 14 ) , . . . . , ( 7 , 8 ) (0,15), (1,14), ...., (7,8) , giving us 8 8 quadruples.

(ii) If ( a , b ) = ( 0 , 1 ) (a,b) = (0,1) then ( c , d ) (c,d) can be ( 1 , 13 ) , ( 2 , 12 ) , . . . , ( 7 , 7 ) (1,13), (2,12), ... ,(7,7) , giving us 7 7 more quadruples.

And continuing with this brute force approach .....

(iii) ( a , b ) = ( 0 , 2 ) ( c , d ) = ( 2 , 11 ) , ( 3 , 10 ) , . . . , ( 6 , 7 ) 5 (a,b) = (0,2) \Longrightarrow (c,d) = (2,11), (3,10), ..., (6,7) \Longrightarrow 5 more quadruples.

(iv) ( a , b ) = ( 0 , 3 ) ( c , d ) = ( 3 , 9 ) , ( 4 , 8 ) , ( 5 , 7 ) , ( 6 , 6 ) 4 (a,b) = (0,3) \Longrightarrow (c,d) = (3,9), (4,8), (5,7), (6,6) \Longrightarrow 4 more quadruples.

(v) ( a , b ) = ( 0 , 4 ) ( c , d ) = ( 4 , 7 ) , ( 5 , 6 ) 2 (a,b) = (0,4) \Longrightarrow (c,d) = (4,7), (5,6) \Longrightarrow 2 more quadruples.

(vi) ( a , b , c , d ) = ( 0 , 5 , 5 , 5 ) (a,b,c,d) = (0,5,5,5) for 1 1 more quadruple.

(vii) ( a , b ) = ( 1 , 1 ) ( c , d ) = ( 1 , 12 ) , ( 2 , 11 ) , . . . , ( 6 , 7 ) 6 (a,b) = (1,1) \Longrightarrow (c,d) = (1,12), (2,11), ... ,(6,7) \Longrightarrow 6 more quadruples.

(viii) ( a , b ) = ( 1 , 2 ) ( c , d ) = ( 2 , 10 ) , . . . . , ( 6 , 6 ) 5 (a,b) = (1,2) \Longrightarrow (c,d) = (2,10), .... ,(6,6) \Longrightarrow 5 more quadruples.

(ix) ( a , b ) = ( 1 , 3 ) ( c , d ) = ( 3 , 8 ) , ( 4 , 7 ) , ( 5 , 6 ) 3 (a,b) = (1,3) \Longrightarrow (c,d) = (3,8), (4,7) ,(5,6) \Longrightarrow 3 more quadruples.

(x) ( a , b ) = ( 1 , 4 ) ( c , d ) = ( 4 , 6 ) , ( 5 , 5 ) 2 (a,b) = (1,4) \Longrightarrow (c,d) = (4,6), (5,5) \Longrightarrow 2 more quadruples.

(xi) ( a , b ) = ( 2 , 2 ) ( c , d ) = ( 2 , 9 ) , . . . , ( 5 , 6 ) 4 (a,b) = (2,2) \Longrightarrow (c,d) = (2,9), ..., (5,6) \Longrightarrow 4 more quadruples.

(xii) ( a , b ) = ( 2 , 3 ) ( c , d ) = ( 3 , 7 ) , ( 4 , 6 ) , ( 5 , 5 ) 3 (a,b) = (2,3) \Longrightarrow (c,d) = (3,7), (4,6), (5,5) \Longrightarrow 3 more quadruples.

(xiii) ( a , b , c , d ) = ( 2 , 4 , 4 , 5 ) (a,b,c,d) = (2,4,4,5) for 1 1 more quadruple.

Finally, we have ( 3 , 3 , 3 , 6 ) , ( 3 , 3 , 4 , 5 ) (3,3,3,6), (3,3,4,5) and ( 3 , 4 , 4 , 4 ) (3,4,4,4) , giving us 3 3 more quadruples.

Adding up all the case results, we end up with a total of 54 \boxed{54} ordered quadruples.

This could help - Substituting w = a w=a , x = b + 1 x=b+1 , y = c + 2 y=c+2 and z = d + 3 z=d+3 transforms the problem into finding ( w , x , y , z ) (w,x,y,z) such that w + x + y + z = 21 w+x+y+z=21 and w < x < y < z w<x<y<z

Pratik Shastri - 6 years, 7 months ago

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