Cassie is going to arrive at the arrivals lounge of an airport between 11 and 12 o'clock. She could arrive at any time with equal probability in that window. She has arranged with her wife Laura to meet and collect her.

If Cassie has to wait more than ten minutes, she'll call a taxi and head home - she's quite impatient. Laura is a little more patient. She arrives between 11 and 11:50 - again with equal probability within that - and waits for 20 minutes. If Cassie doesn't show up, Laura will head home, probably a little frustrated.

What is the probability that Laura picks Cassie up successfully?