A decameter is 10 meters. A kilometer is 1,000 meters. There are 100 decameters in 1 kilometer, or a 100/1 conversion ratio. To convert kilometers to decameters, multiply by 100. 79.8 kilometers x 100 kilometers/decameters = 7,980 decameters.
The Strait of Gibraltar is about 7-8 miles wide. However, Spain actually controls Ceuta, which is the African peninsula on the Strait of Gibraltar. So, Spain actually borders Morocco directly at the Strait of Gibraltar (although there is a no-man's land of roughly 100-200 feet that separates them).