15 nha dan 

duyet nhe

a ) X : 4 = 3845 - 2600                                           b) X x 3 = 3600 - 1200

    X : 4 = 1245                                                            X x 3 =2400

    X = 1245 x 4 =4980                                                  X = 2400 : 3 = 800

5/7=30/42;6/7=36/42

=> 5 p số đó là : 31/42;32/42;33/42;34;42;35/42