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In a class of 52 students 16 are science students.
If 1/3 of the boys and 1/4 of the girls are a Science students, how many boys are in the class?
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To make an impression (and to shake your mind), I can solve the problem differently.
Let x be of the boys and let y be of the girls in the class.
Then the number of boys is 3x and the number of girls is 4y.
So we have two equations
x + y = 16 (1)
3x + 4y = 52. (2)
It is equivalent to (after multiplying the first equation by 3)
3x + 3y = 48 (3)
3x + 4y = 52. (4)
which implies (subtracting equation (3) from (4) )
y = 52 - 48 = 4.
Thus the number of girls in the class is 4*4 = 16, and the boys are the rest population 52 - 16 = 36.
Solved.
What I did (introduced the unknowns by a non-standard way) may work productively and facilitate solutions in many other cases.