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 1∼1001 \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+10⩽1013 z+10 \leqslant 101, i.e., z⩽30z \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+10⩾1023 z+10 \geqslant 102, i.e., 31⩽z⩽6331 \leqslant z \leqslant 63, the number of ternary tuples satisfying x+y=3z+10x+y=3 z+10 is
T=∑k=3163(191−3k)=∑k=133[191−3(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+T−A−B−C=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.