There are 10 centimeters per 1 decimeter. This means that 1 centimeter equals 0.1 decimeters. Multiply 0.1 by 27 to find how many decimeters are in 27 centimeters. 0.1 • 27 = 2.7 So there are 2.7 decimeters in 27 centimeters.
Since the coral is at -3 feet and the cave is at -15 feet we can find out that the difference between the two is 12 feet by doing -15-3 = 12. Hope this helps.