The diameter of a ping pong ball is 4.0cm. find the length in kilometers of a row of 6.02×1023 ping pong balls.

The length is determined by multiplying the number of balls with the diameter. The answer that would come out of this is in units of centimeters.

    L = (6.02 x 10^23 balls)(4 cm/ball)
    L = 2.408 x 10^24 cm

Then, we use the proper conversion factor to convert centimeter to kilometer.

     L = (2.408 x 10^24 cm)(1 m/100 cm)(1 km/1000 m)
       L = (2.408 x 10^24)(10^5)
        

The answer to this item is 2.408 x 10^19 km. 

Let ping pong ball be called PPB
 To solve this problem you must know the following unit change
 1km = 1000m
 1m = 100cm
 Then, applying the change of units, we have that the length in kilometers is:
 6.02x10 ^ 23PPB x 4.0cm / 1PPB x 1m / 100cm x 1km / 1000m = 2.41 x 10 ^ 19 km

Sorry can u repeat that?