Olympiad Maths Prep

Track / Stage 5 / 194 of 400 #794 of 2000

Problem 794

AIME late
Combinatorics Difficulty 5.5 Find the answer

[ Riddles [ Evenness and Oddness ]\begin{gathered}{\left[\begin{array}{l}\text { Riddles } \\ {[\text { Evenness and Oddness }}\end{array}\right]}\end{gathered}

Decode the riddle

44H×HH+4H4H4HHHHHHH \begin{array}{r} \begin{array}{r} 44 H \\ \times H H \end{array} \\ +\begin{array}{r} 4 H 4 H \\ 4 H H \end{array} \\ \hline H H H H H \end{array}

All digits marked with the letter E are even (not necessarily equal); all digits marked with the letter H\mathrm{H} are odd (also not necessarily equal).

Official solution

For the convenience of further reasoning, let's replace all even numbers with vowels and all odd numbers with consonants, keeping in mind that different letters can correspond to the same digit. The riddle will take the following form:

×EB×CD F FUG +YHK YMNPR  \begin{aligned} \times E B \\ \times \quad C D \\ \hline \text { F FUG } \\ +Y H K \\ \hline \text { YMNPR } \end{aligned}

Note that the numbers AEB and YHK differ in the second digit, so C>1 \mathrm{C} > 1 . If C>3 \mathrm{C} > 3 or A>2 \mathrm{A} > 2 , the product AEB \cdot C will be a four-digit number, so C=3, A=2 \mathrm{C} = 3, \mathrm{~A} = 2 . Therefore, I is no more than 2, which means it is equal to 2. From this, it follows that D=9 \mathrm{D} = 9 (with a smaller value of D \mathrm{D} , the expression AEB \cdot D will be less than 2100, while IFUG >2100 > 2100 ). Y=8 \mathrm{Y} = 8 (otherwise, the entire product will not be a five-digit number). E=8 \mathrm{E} = 8 (if E<8 \mathrm{E} < 8 , then YHK<810 \mathrm{YHK} < 810 ). As B changes from 1 to 9, the number IFUG will change from 2529 to 2601, so F=5 \mathrm{F} = 5 , and B<9 \mathrm{B} < 9 . H=5 \mathrm{H} = 5 (as B changes from 1 to 7, the number YHK will change from 843 to 861). K=5 \mathrm{K} = 5 (otherwise, the number 85 K 85 \mathrm{~K} will not be divisible by 3).

## Answer

285×39=11115 285 \times 39 = 11115

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.