Hi, Could someone please explain the substitution in this question and where the 5dy comes from? Many thanks, Tom
X is the random variable of total claim amount and Y is the random variable of claim amt. paid by the reinsurer. Here, X~Gamma(a,L) Retention limit is 80%, means the reinsurer is paying 20% proportion of each claim. So, Y= 0.20 X Take the pdf of gamma dist. and substitute Y= 0.20X → X= 1/0.20 Y → X= 5Y and so, dx= 5dy. Try by yourself, you will get it.