Let a random variable X represent a coffee stand's revenue from a single customer. Say the shop sells small, medium, and large coffee for $3, $4, and $5, respectively. Customers buy a small coffee 30% of the time, a medium coffee 50% of the time, and a large coffee 20% of the time. What is the expected value of the coffee stand's revenue from a single customer?
$3.90
$4.00
$4.10
$4.20