The yield chain

You will write three small functions that follow a product through the steps of downstream processing.

The idea. Each step keeps a share of what it gets. The overall yield is the product of the shares.

Your tasks.

1. Finish overall_yield. It takes a list of shares, each between 0 and 1, and gives their product.
2. Finish finished_amount. It gives the amount of the finished product from the amount in the culture and the list of shares.
3. Finish start_needed. It gives the amount that the culture must hold, to end with a target amount.