Maths Olympiad Prep

Track / Stage 5 / 53 of 400 #653 of 1964

Problem 653

AIME late
Algebra Difficulty 5.1 Find the answer

21. 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. Spacing and $ signs are ignored.

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 and solution reproduced as published; topic, difficulty and ordering added by this site.