Aturtle swims at a speed of 15,840 yd per h. a girl swims at a speed of 138 cm per s. 1 dekameter (dam) = 11 yd 1 dam = 1000 cm how much faster does the turtle swim than the girl in dekameters per minute?

There are several information's already given in the question. Let us write them down first and then concentrate on the problem in hand.

11 yard = 1 dam
Then
11 yard = 1000 cm
So
15840 yard = (1000/11) * 15840 cm
                   = 1440000 cm
Distance covered by the turtle in 1 hour = 15, 840 yard
Distance covered by the turtle in 60 minutes = 1440000 cm
Distance covered by the turtle in 1 minute = 1440000/60 cm
                                                                   = 24000 cm
Distance covered by the turtle in 60 second = 24000 cm
Distance covered by the turtle in 1 second = 24000/60 cm
                                                                    = 400 cm
So
 The speed at which the turtle swims can also be written as 400 cm per second.
 The speed at which the girl swims is 138 cm per second.
 The speed by which
the turtle swims faster than the girl = (400 - 138) cm/sec
                                                        = 262 cm/sec