Đổi: \(60\%=\frac{3}{5}\)

a) Số học sinh thích bóng bàn chiếm: \(1-\left(\frac{3}{5}+\frac{1}{5}\right)=\frac{1}{5}\)( số học sinh cả lớp )

Số học sinh cả lớp là: \(7:\frac{1}{5}=35\)( học sinh )

b) Tỉ số phần trăm giữa học sinh thích bóng bàn và bóng chuyền là: \(\frac{1}{5}:\frac{1}{5}.100\%=100\%\)

Help me !

A=7.(1/10+1/10^2+.....+1/10^100)

10A=7.(10/10+10/10^2+....+10/10^100)
10A=7.(1+1/10+....+1/99)           9A=7.(ans-A)              9A=7.(1-1/100)              9A=7-7/100        A=(7-7100):9

\(\frac{x-1}{3}+\frac{1}{y}=\frac{-1}{6}\) 

\(\frac{\left(x-1\right)y}{3y}+\frac{3}{3y}=\frac{-1}{6}\) 

\(\frac{\left(x-1\right)y+3}{3y}=\frac{-1}{6}\) 

\(\frac{\left(x-1\right)y}{y}=\frac{\left(-1\right)-3}{6:3}\) 

\(x-1=-2\)

\(x=\left(-2\right)+1\)

\(x=-1\)

\(\frac{x-1}{3}+\frac{1}{y}=\frac{-1}{6}\)

 \(\frac{\left(x-1\right)y}{3y}+\frac{3}{3y}=\frac{-1}{6}\)

\(\frac{\left(x-1\right)y+3}{3y}=\frac{-1}{6}\) 

\(x-1=\frac{\left(-1\right)-3}{6:3}\)

\(x-1=-2\)

\(x=\left(-2\right)+1\) 

\(x=-1\)

\(a,\frac{-2}{3}\div\left(\frac{1}{2}-3x\right)=\frac{5}{3}\)

\(\frac{1}{2}-3x=\frac{-2}{3}\div\frac{5}{3}\)

\(\frac{1}{2}-3x=\frac{-2}{5}\)

\(3x=\frac{1}{2}-\frac{-2}{5}\)

\(3x=\frac{9}{10}\)

\(x=\frac{9}{10}\div3=\frac{3}{10}\)

\(b,\frac{1}{7}\left|x\right|-\frac{4}{5}=\frac{1}{5}\)

\(\frac{1}{7}\left|x\right|=\frac{1}{5}+\frac{4}{5}=1\)

\(\left|x\right|=1\div\frac{1}{7}=7\)

\(\Leftrightarrow x=\pm7\)

-2/3:(1/2-3x)=5/3

(1/2-3x)=5/3.-3/2

(1/2-3x)=-5/2

3x=-5/2+1/2

3x=-3/2

x=-3/2:3/1

x=-3/2.1/3

x=-1/2

(2/9.3/5+1/3.3/5).(1/3-5/8)

(6/35+1/5).(1/3-5/8

(6/35+7/35).(1/3-5/8

13/35.(1/3-5/8)

13/35.-7/24

-91/840

\(\left(\frac{2}{9}:\frac{5}{3}+\frac{1}{3}:\frac{5}{3}\right).\left(\frac{1}{3}-\frac{5}{8}\right)\)

\(=\left[\left(\frac{2}{9}+\frac{1}{3}\right).\frac{3}{5}\right].\frac{-7}{24}\)

\(=\left(\frac{5}{9}.\frac{3}{5}\right).\frac{-7}{24}\)

\(=\frac{1}{3}.\frac{-7}{24}\)

\(=\frac{-7}{72}\)