x/10 = y/5 va y/2=z/3

suy ra x/20 =y/10;y/10=z/15.

suy ra x/20=y/10=z/15=4z/60 = x+4z/20 + 60 = 320/80=4 

suy ra x= 80.; y=40; z=60

ta goi xy lan luot la so quyen sach quyen gop cua moi lop 7A va 7B( x, y thuoc N* )

x/32= y/36 = x-y/32-36=8/4=2

suy ra x=32x2 =64 ; y=36 x2 =72

Vay lop 7A quyen gop 64 quyen sach.

7B quyen gop 72 quyen sach.

a) k= a x b = 3 x (-10) = -30

b) a x b =-30. a=2 suy ra -30 : 2 =- 15

a) a/b = c/d = e/f =a+c+e/b+d+f

b) x+y/4+7=55/11=5

x/4 = 5 => x = 4 x 5 =20.

y/7 = 5 => y = 7 x 5 = 35.

Vay x= 20; y = 35.

a) 1/5 : 1/2 = 1 : 2,5

b) 5/3= x/9 

x= 5 x 9 : 3

x= 15