100 + 500 : 5 - 40

   =  100 + ( 500 : 5 ) - 40 

   =  100 + 100 - 40

    = 100 + 60 

    =  160

Number of female students is : 

   40 x \(\dfrac{2}{5}\)= 16 ( student )

 Number of  male students is :

    40 - 16 = 24 ( student )

         Answer : 24 student

\(\dfrac{1}{2}+\dfrac{3}{4}\)

\(=\dfrac{1\times2}{2\times2}+\dfrac{3}{4}\)

\(=\dfrac{2}{4}+\dfrac{3}{4}\)

\(=\dfrac{5}{4}\)

( x + 1 ) . ( 1 + 1999 ) = 4000

 (x + 1 ) . 2000 = 4000

   x + 1 = 4000 : 2000

   x + 1 = 2 

   x = 2 - 1 

  x = 1 

That worker makes 3 tools out of time is

   12 hours - 7 hours 30 minute = 4 hours 30 minute 

                                                   = 4,5 hours 

That worker makes a product time out is

    4,5 : 3 = 1,5 ( hours )

         Answer : 1,5 hours

100 +( 500 : 5) - 40 

= 100 + 100 - 40 

= 200 - 40 

= 160

\(\dfrac{5}{8}+\dfrac{9}{8}=\dfrac{5+9}{8}=\dfrac{14}{8}=\dfrac{7}{4}\)

\(\dfrac{6}{7}-\dfrac{3}{7}=\dfrac{3}{7}\)

( x + 1 ) . ( 1 + 1999 ) = 4000

x + 1 = 4000 : ( 1 + 1999 )

x  + 1 = 4000 : 2000

  x + 1 = 2 

   x = 2 - 1 

  x = 1