.
Add (mentally) one person to the band.
Then the number of persons is a multiple of 2, is a multiple of 5 and is a multiple of 7.
The smallest integer positive number with such properties is 2*5*7 = 70.
Now subtract (detach) this additional person, whom you added at the very beginning.
Thus you will get the ANSWER : least number of people in the marching band is 70-1 = 69.
CHECK. 69 is 1 short to be arranged in pairs;
is 1 short to be arranged by 5s,
and is 1 short to be arranged by 7s.
Solved and completed.
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The tutor @greenestamps misread the problem and gave wrong answer.
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To see many other similar solved problems, look into the lesson
- The number that leaves a remainder 1 when divided by 2, by 3, by 4, by 5 and so on until 9
in this site.
The Problem 2 of that lesson is an exact analogue to your problem in this post.