25 points!! Probability question.
This is from Math-a-thon, pls solve this problem to help me help children with cancer...
Destructimous has 7 sonic blasts and 13 heat rays. It also has a 2/7 chance of using its ultra power blast. Luckily, Octagon’s sheild reflected that blast and stopped all the robots. How many ultra power bots did Destructimous have? Pls show/ explain your work. (i know the answer i just dont know how to find it).