Maths Olympiad Prep

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Algebra Difficulty 4.9 AIME Find the answer

The sum of the digits of the time 19 minutes ago is two less than the sum of the digits of the time right now. Find the sum of the digits of the time in 19 minutes. (Here, we use a standard 12-hour clock of the form hh:mm.)

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let's say the time 19 minutes ago is hh hours and mm minutes, so the sum of the digits is equivalent to h+mmod9h+m \bmod 9. If m40m \leq 40, then the time right now is hours and m+19m+19 minutes, so the sum of digits is equivalent \bmod 9 to h+m+19h+m+1(mod9)h+m+19 \equiv h+m+1(\bmod 9), which is impossible. If m>40m>40 and h<12h<12, then the time right now is h+1h+1 hours and m41m-41 minutes, so the sum of digits is equivalent to h+m40h+m+5(mod9)h+m-40 \equiv h+m+5(\bmod 9), which is again impossible. Therefore, we know that h=12h=12 and m>40m>40. Now, the sum of the digits 19 minutes ago is 3+s(m)3+s(m), where s(n)s(n) is the sum of the digits of nn. On the other hand, the sum of the digits now is 1+s(m41)1+s(m-41), meaning that 4+s(m)=s(m41)4+s(m)=s(m-41). The only mm that satisfies this is m=50m=50, so the time right now is 1:09. In 19 minutes, the time will be 1:28, so the answer is 11.

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