Maths Olympiad Prep

Track / Stage 5 / 97 of 400 #697 of 1964

Problem 697

AIME late
Algebra Difficulty 5.1 Find the answer

Among the 100 integers from 11001 \sim 100, arbitrarily select three different numbers to form an ordered triplet (x,y,z)(x, y, z). Find the number of triplets that satisfy the equation x+y=3z+10x+y=3z+10.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

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Official solution

(1) When 3z+101013 z+10 \leqslant 101, i.e., z30z \leqslant 30, the number of ternary tuples satisfying x+y=3z+10x+y=3 z+10 is
S=k=130(3k+9)=1665 S=\sum_{k=1}^{30}(3 k+9)=1665 \text{. }
(2) When 3z+101023 z+10 \geqslant 102, i.e., 31z6331 \leqslant z \leqslant 63, the number of ternary tuples satisfying x+y=3z+10x+y=3 z+10 is
T=k=3163(1913k)=k=133[1913(k+30)]=1650. \begin{aligned} T & =\sum_{k=31}^{63}(191-3 k) \\ & =\sum_{k=1}^{33}[191-3(k+30)]=1650 . \end{aligned}

Now consider the case where x,y,zx, y, z are equal.
First, x,y,zx, y, z cannot all be equal.
If x=yx=y, then the number of ternary tuples satisfying x+y=3z+10x+y=3 z+10 is A=31A=31.

If x=zx=z, then the number of ternary tuples satisfying x+y=3z+10x+y=3 z+10 is B=45B=45.

If y=zy=z, then the number of ternary tuples satisfying x+y=3z+10x+y=3 z+10 is C=45C=45.
In summary, the number of ternary tuples is
S+TABC=3194. S+T-A-B-C=3194 .

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.