Let $Y_{1}$ denote the weight (in tons) of a bulk item stocked by a supplier at the beginning of a week
and suppose that $Y_{1}$ has a uniform distribution over the interval $0 \leq y_{1} \leq 1$. Let $Y_{2}$ denote the amount (by weight) of this item sold by the supplier during the week and suppose that $Y_{2}$ has a uniform distribution over the interval $0 \leq y_{2} \leq y_{1},$ where $y_{1}$ is a specific value of $Y_{1}$
a. Find the joint density function for $Y_{1}$ and $Y_{2}$
b. If the supplier stocks a half-ton of the item, what is the probability that she sells more than a quarter-ton?
c. If it is known that the supplier sold a quarter-ton of the item, what is the probability that she had stocked more than a half-ton?