9514 1404 393
Answer:
13
Step-by-step explanation:
The number is 1 more than the least common multiple of 3 and 4. That multiple is 12, so the number of stairs is 12+1 = 13.
Martha's school staircase has 13 steps, minimum.
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Additional comment
The real minimum is 1, not 13. We assume that we're interested in the non-trivial solution. (In a public building, staircases between floors often have 14 steps. 13 steps might attract the attention of those who are superstitious.)
