there are 3 boxes with 10 chocolates in the first 5 of them are with strawberries , 10 chocolates in the second 4 of them are with strawberries 10 chocolates n the third and 3 of them are with strawberries

if we chose randomly one box and then randomly one chocolate how much is the probability to chose one that is with strawberries?

how to solve this? witch formula should i use ?

thanks for any reply