How many ordered quadruples ( a , b , c , d ) , where a , b , c , d are non-negative integers and a ≤ b ≤ c ≤ d , satisfy the equation a + b + c + d = 1 5 ?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
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 ) then ( c , d ) can be ( 0 , 1 5 ) , ( 1 , 1 4 ) , . . . . , ( 7 , 8 ) , giving us 8 quadruples.
(ii) If ( a , b ) = ( 0 , 1 ) then ( c , d ) can be ( 1 , 1 3 ) , ( 2 , 1 2 ) , . . . , ( 7 , 7 ) , giving us 7 more quadruples.
And continuing with this brute force approach .....
(iii) ( a , b ) = ( 0 , 2 ) ⟹ ( c , d ) = ( 2 , 1 1 ) , ( 3 , 1 0 ) , . . . , ( 6 , 7 ) ⟹ 5 more quadruples.
(iv) ( a , b ) = ( 0 , 3 ) ⟹ ( c , d ) = ( 3 , 9 ) , ( 4 , 8 ) , ( 5 , 7 ) , ( 6 , 6 ) ⟹ 4 more quadruples.
(v) ( a , b ) = ( 0 , 4 ) ⟹ ( c , d ) = ( 4 , 7 ) , ( 5 , 6 ) ⟹ 2 more quadruples.
(vi) ( a , b , c , d ) = ( 0 , 5 , 5 , 5 ) for 1 more quadruple.
(vii) ( a , b ) = ( 1 , 1 ) ⟹ ( c , d ) = ( 1 , 1 2 ) , ( 2 , 1 1 ) , . . . , ( 6 , 7 ) ⟹ 6 more quadruples.
(viii) ( a , b ) = ( 1 , 2 ) ⟹ ( c , d ) = ( 2 , 1 0 ) , . . . . , ( 6 , 6 ) ⟹ 5 more quadruples.
(ix) ( a , b ) = ( 1 , 3 ) ⟹ ( c , d ) = ( 3 , 8 ) , ( 4 , 7 ) , ( 5 , 6 ) ⟹ 3 more quadruples.
(x) ( a , b ) = ( 1 , 4 ) ⟹ ( c , d ) = ( 4 , 6 ) , ( 5 , 5 ) ⟹ 2 more quadruples.
(xi) ( a , b ) = ( 2 , 2 ) ⟹ ( c , d ) = ( 2 , 9 ) , . . . , ( 5 , 6 ) ⟹ 4 more quadruples.
(xii) ( a , b ) = ( 2 , 3 ) ⟹ ( c , d ) = ( 3 , 7 ) , ( 4 , 6 ) , ( 5 , 5 ) ⟹ 3 more quadruples.
(xiii) ( a , b , c , d ) = ( 2 , 4 , 4 , 5 ) for 1 more quadruple.
Finally, we have ( 3 , 3 , 3 , 6 ) , ( 3 , 3 , 4 , 5 ) and ( 3 , 4 , 4 , 4 ) , giving us 3 more quadruples.
Adding up all the case results, we end up with a total of 5 4 ordered quadruples.
This could help - Substituting w = a , x = b + 1 , y = c + 2 and z = d + 3 transforms the problem into finding ( w , x , y , z ) such that w + x + y + z = 2 1 and w < x < y < z
Problem Loading...
Note Loading...
Set Loading...
Same as of Brian Charlesworth