Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: the base of a triangle is 15 cm and height is 12 cm . the height of another triangle of double the area having the base 20 cm is :
18 cm'
24 cm.