C = -1/2 +3/21 +-2/6 + -5/30

           = -1/2 + 1/7 ( rút gọn 3/21 cho 3 ) + -1/3 (rút gọn cho 2 ) + -1/6 (rút gọn cho 6)

          =( -1/2 +  -1/3 ) + 1/7 + -1/6

        =  -5/6  + 1/7 + -1/6

           = (-5/6 + -1/6 ) + 1/7

            = -1 +1/7

           = -7/7 + 1/7

          = -6/7

1/ It (rain) is raining heavily now. You should (stay) stay at home and (read) read books.

2/ What are you (do) doing at the moment? - I (write) am writing an essay.

3/ Where will Lan and Ba (go) go tomorrow? They (visit) will visit the Museum.

4/ You (have) will have Geography next Friday.

5/ What about (play) playing a game of chess?

6/ It takes her 10 minutes (make) to make this toy.

7/ She (be) will be ten on her next birthday.

 8/ I (meet) will meet you soon.

9/ Why don’t we (hold) hold a party?