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S = Numbers of students
Percent of boys.
100 % - 55 % = 45 % = 45 / 100
45 % S = 18
45 S / 100 = 18 Multiply both sides by 100
45 S = 18 * 100
45 S = 1800 Divide both sides by 45
S = 1800 / 45 = 40