In my grapefruit tree, there are yellow grapefruits and orange grapefruits.

I have noticed that 90% of all yellow grapefruits are sour and the other 10% of yellow grapefruits are sweet.

I have noticed that 50% of the orange grapefruits are sour and 50% of the orange grapefruits are sweet.

80% of the grapefruits in my tree are orange and the remaining 20% are yellow.

My daughter ate a grapefruit from the tree and told me it was sour. What is the probability that she picked a yellow grapefruit?

(Explain your answer.)