Maths Olympiad Prep

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Number theory Difficulty 4.7 AIME Find the answer Canada

In the six-digit number 1ABCDE1ABCDE, each letter represents a digit. Given that 1ABCDE×3=ABCDE11ABCDE \times 3 = ABCDE1, the value of A+B+C+D+EA+B+C+D+E is

Pick one

Solutions — 2

Solution 1

The units digit of the product 1ABCDE×31ABCDE\times3 is 1, and so the units digit of E×3E\times3 must equal 1.

Therefore, the only possible value of EE is 7.

Substituting E=7E=7, we get

[[IMAGE0]]

Since 7×3=217\times3=21, 2 is carried to the tens column.

Thus, the units digit of D×3+2D\times3+2 is 7, and so the units digit of D×3D\times3 is 5.

Therefore, the only possible value of DD is 5.

Substituting D=5D=5, we get

[[IMAGE1]]

Since 5×3=155\times3=15, 1 is carried to the hundreds column.

Thus, the units digit of C×3+1C\times3+1 is 5, and so the units digit of C×3C\times3 is 4.

Therefore, the only possible value of CC is 8.

Substituting C=8C=8, we get


[[IMAGE2]]

Since 8×3=248\times3=24, 2 is carried to the thousands column.

Thus, the units digit of B×3+2B\times3+2 is 8, and so the units digit of B×3B\times3 is 6.

Therefore, the only possible value of BB is 2.

Substituting B=2B=2, we get


[[IMAGE3]]

Since 2×3=62\times3=6, there is no carry to the ten thousands column.

Thus, the units digit of A×3A\times3 is 2.

Therefore, the only possible value of AA is 4.

Substituting A=4A=4, we get

[[IMAGE4]]

Checking, we see that the product is correct and so A+B+C+D+E=4+2+8+5+7=26A+B+C+D+E=4+2+8+5+7=26.

Solution 2

Points P,Q,R,S,TP,Q,R,S,T divide the bottom edge of the park into six segments of equal length, each of which has length 600÷6=100600\div6=100 m.

If Betty and Ann had met for the first time at point QQ, then Betty would have walked a total distance of 600+400+4×100=1400600+400+4\times100=1400 m and Ann would have walked a total distance of 400+2×100=600400+2\times100=600 m.

When they meet, the time that Betty has been walking is equal to the time that Ann has been walking and so the ratio of Betty’s speed to Ann’s speed is equal to the ratio of the distance that Betty has walked to the distance that Ann has walked.

That is, if they had met for the first time at point QQ, the ratio of their speed’s would be 1400:6001400:600 or 14:614:6 or 7:37:3.

Similarly, if Betty and Ann had met for the first time at point RR, then Betty would have walked a total distance of 600+400+3×100=1300600+400+3\times100=1300 m and Ann would have walked a total distance of 400+3×100=700400+3\times100=700 m.

In this case, the ratio of their speed’s would be 1300:7001300:700 or 13:713:7.

When Betty and Ann actually meet for the first time, they are between QQ and RR.

Thus Betty has walked less distance than she would have had they met at QQ and more distance than she would have had they met at RR.

That is, the ratio of Betty’s speed to Ann’s speed must be less than 7:37:3 and greater than 13:713:7.

We must determine which of the five given answers is a ratio that is less than 7:37:3 and greater than 13:713:7.

One way to do this is to convert each ratio into a mixed fraction.

That is, we must determine which of the five answers is less than 7:3=73=2137:3=\frac73=2\frac13 and greater than 13:7=137=16713:7=\frac{13}{7}=1\frac67.

Converting the answers, we get 53=123,94=214,116=156,125=225,\frac53=1\frac23, \frac94=2\frac14, \frac{11}{6}=1\frac56, \frac{12}{5}=2\frac25, and 177=237\frac{17}{7}=2\frac37.

Of the five given answers, the only fraction that is less than 2132\frac13 and greater than 1671\frac67 is 2142\frac14.

If Betty and Ann meet for the first time between QQ and RR, then the ratio of Betty’s speed to Ann’s speed could be 9:49:4.

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.