Question

Danica drove her new car on a trip for a whole number of hours, averaging $55$ miles per hour. At the beginning of the trip, $abc$ miles was displayed on the odometer, where $abc$ is a $3$ -digit number with $a\ge1$ and $a+b+c\le7.$ At the end of the trip, the odometer showed $cba$ miles. What is $a^2+b^2+c^2?$

$26$

$27$

$36$

$37$

$41$

Answer: D

Concepts:digits divisibility

Difficulty rating: 1540

Solution:

We know that the difference of the numbers $cba$ and $abc$ is equal to: $100c + 10b+a - 100a - 10b-c$ $= 99(c-a)$ We know that this number also must be a multiple of $55.$ As $\gcd(55,99)$ is $11,$ we know that $c-a$ is a multiple of $5,$ and $c > a.$

This makes $a = 1, b = 0, c = 6$ the only possible value with $a+ b+c \leq 7$ as every other combination has $a+b+c > 7.$ As such, $a^2+b^2+c^2 = 37.$

Thus, the correct answer is D .

2014 AMC 10B Problem 14

Answer: D

Difficulty rating: 1540

Solution:

Problem 14 in Other Years