a) There are :7 x 7 x 6 x 5 = 1470 numbers

b) There are :4 x 4 x 3 x 2 = 96 even numbers

There are : (200 - 1) : 1 + 1 = 200 numbers from 1 to 200

We have a sequence of two-digit numbers are as follows :

2 ; 12 ; 22 ; 32 ; ....... ; 192

Apply a formula to count numbers , we have :

(192 - 2) : 10 + 1 = 20 

There are also numbers with 2 digits such as

20 ; 21 ; 23 ; 24 ; .... ; 29 ; 120 ; 121 ; 122 ; 123 ; ...... 129

And there are have 18 also numbers .

So , from these information , we have 38 numbers have digit 2

We have :

200 - 38 = 162 (numbers)

There are 162 numbers from 1 to 200 do not have the digit 2 

Change : half an hour = 0,5 hour

The speed of the car is :

   ( 25 - 5 ) : 0,5 = 40 ( km / hour )

              Answer : 40 km / hour

Numbers sweet is the biggest that he can get from the shop is : 

                                         30 : 3 = 10 (sweet)

                                                 Answer : 10 sweet

The distance between the houses is: 3000: 100 = 30 (minute)
Homeownership is: 3000: 150 = 20 (minutes)
Average by John in a minute: 3000: (20 + 30) = 60 (minute)
                                              Answers:60 minute

The distance between the houses is: 3000: 100 = 30 (minute)
Homeownership is: 3000: 150 = 20 (minutes)
Average by John in a minute: 3000: (20 + 30) = 60 (minute)
Answers:60 minute

The distance between the houses is: 3000: 100 = 30 (minute)
Homeownership is: 3000: 150 = 20 (minutes)
Average by John in a minute: 3000: (20 + 30) = 60 (minute)
                                              Answers:60 minute

Numbers sweet is the biggest that he can get from the shop is : 

                                         30 : 3 = 10 (sweet)

                                                 Answer : 10 sweet 

Change : half an hour = 0,5 hour

The speed of the car is :

   ( 25 - 5 ) : 0,5 = 40 ( km / hour )

              Answer : 40 km / hour

The distance between the houses is: 3000: 100 = 30 (minute)
Homeownership is: 3000: 150 = 20 (minutes)
Average by John in a minute: 3000: (20 + 30) = 60 (minute)

Answers:60 minute

a ) 6 : 9 = \(\dfrac{6}{9}\)

b ) 7 : 4 = \(\dfrac{7}{4}\)

c ) 12 : 5 = \(\dfrac{12}{5}\)

d ) 9 : 19 = \(\dfrac{9}{19}\)

e ) 7 : 13 = \(\dfrac{7}{13}\)

Numbers students femsla are in class is : 

                      60 x 3/5 = 36 (students)

Numbers students male are in class is : 

                      60 - 36 = 24 (students)

                         Answer : 24 students

Average is :

  ( 45 + 126 + 51 + 549 + 83 + 84 + 89 + 169 + 545 ) : 8 = 206,5

                                                                                Answer : 206,5

Number of female is :       60 x 3 : 5 = 36 ( student )

Number of male is :           60 - 36 = 24 ( student )

                                                    Answer : 24 student

Average is :

  ( 45 + 126 + 51 + 549 + 83 + 84 + 89 + 169 + 545 ) : 8 = 206,5

                                                                                Answer : 206,5

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

 (x + 1 ) . 2000 = 4000

   x + 1 = 4000 : 2000

   x + 1 = 2 

   x = 2 - 1 

  x = 1 

Number of female students is :

  40 x 2525= 16 ( student )

Number of male students is :

   40 - 16 = 24 ( student )

      Answer : 24 student

Numbers of terms is :

   ( 1000 - 1 ) : 2 + 1 = 505,5

The sum is : 

   ( 1000 + 1 ) x 505,5 : 2 = 250500,25

                                   Answer : 250500,25

Number of female students is:         \(50\cdot\dfrac{2}{5}\)=20 ( students )

Number of male students is :   50 - 20 = 30 ( students )

                                               Answer : 30 students