4.2 km 1 mile 5,280 ft 22,176
* ---------- * ----------- = ----------- = 13,782.5 feet
1.609 km 1 mile 1.609
Using the conversion factors and setting the problem up so that you have km on top and bottom (so that they will cancel), miles on top and bottom (so that they will cancel), your answer will be left in feet.
Multiply across the numerator, then across then denominator. Finally simplify the fraction at the end.