Question

On the playground, if the children are made to stand for drill either 20 to a row or 25 to a row, all rows are complete and no child is left out. What is the lowest possible number of children in that school?

Answer 1

If the children are to stand in rows such that all rows are complete and no children is left out, then the number of children in the school must be in multiples of both 20 and 25. So, the lowest possible number of children in the school is the LCM of 20 and 25.
Multiples of 20: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200,...
Multiples of 25: 25, 50, 75, 100, 125, 150, 175, 200,...
Common multiples of 20 and 25: 100, 200,...
∴ Lowest of possible number of children in that school = LCM of 20 and 25 = 100
Thus, the lowest of possible number of children in that school is 100.