4/9 of the students are boys thus 5/9 students are girls.
Let " x " be the number of all students.
So we have :
5/9 × x = 90
Multiply both sides by 9/5
5/9 × 9/5 × x = 90 × 9/5
1 × x = 9 × 9 × 10 × 1/5
x = 81 × 2
x = 162
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number of boys = 4/9 × x
n o boys = 4/9 × 162
n o boys = 162/9 × 4
n o boys = 18 × 4
n o boys = 72
