a + b = 70
a = [tex]\frac{3}{4}[/tex] · b
__________
a = ?
b = ?
__________
a + b = 70
[tex]\frac{3}{4}[/tex] · b + b = 70
(3b + 4b) : 4 = 70
7b = 70 · 4
7b = 280
b = 280 : 7
b = 40
a = [tex]\frac{3}{4}[/tex] · b
a = [tex]\frac{3}{4}[/tex] · [tex]\frac{40}{1}[/tex]
a = 30