Question

In a certain alien world, the maximum running speed $v$ of an organism is dependent on its number of toes $n$ and number of eyes $m.$ The relationship can be expressed as $v = k n^a m^b$ centimeters per hour, where $k,$ $a,$ and $b$ are integer constants. In a population where all organisms have $5$ toes, $\log v = 4 + 2\log m;$ and in a population where all organisms have $25$ eyes, $\log v = 4 + 4\log n,$ where the logarithms are base $10.$ What is $k + a + b?$

$20$

$21$

$22$

$23$

$24$

Solution:

Taking logarithms, $\log v = \log k + a\log n + b\log m.$

With $n = 5,$ this reads $\log v = (\log k + a\log 5) + b\log m,$ matching $4 + 2\log m,$ so $b = 2$ and $\log k + a\log 5 = 4.$

With $m = 25,$ it reads $\log v = (\log k + b\log 25) + a\log n,$ matching $4 + 4\log n,$ so $a = 4$ and $\log k + 2\log 25 = 4.$

Then $\log k = 4 - \log 625 = \log\dfrac{10000}{625} = \log 16,$ so $k = 16.$ Hence $k + a + b = 16 + 4 + 2 = 22.$

Thus, the correct answer is C.

2025 AMC 12A Problem 7

Solution:

Problem 7 in Other Years