For a certain section of pine forest, the number Y of diseased trees per acre has a Poisson distribution with mean lambda=10. The disased trees are sprayed with an insecticide at a cost of 3 dollars per tree, plus a fixed overhead cost for equipment rental of 50 dollars. Letting C denote the total spraying cost for a randomly selected acre, find the expected value and variance for C.
Expected value is easy to find: 80
I'm stuck on Variance? Why is variance 90?