Sally is helping her father dig worms for fishing. "That's exactly enough." he says as she places a worm in his tin. "How many do you need?" she enquires. "I use 4 worms on my hook and reckon on having to replace an average of one of these each time I cast. I cast 12 times an hour. Today I will fish 5 pools spending an hour at the first 4 and an hour and a half at the fifth. I always discard the worms on my hook when walking between pools." How many worms are in the tin?