Answer :

mathlovingguy
Number of boys are 30

Assume number of boys (X) , Number of Girls (Y)
X + Y = 70                       (1)      
X/Y = 3/4
Y = 4/3X                          (2)

replacing the value of Y in (2) in number (1)

X + 4X/3 = 70
7X/3 = 70
7X = 210
X = 30

so number of boys = 30 boys

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