I'm stuck on this math problem: Mr. Bodhi is transporting some animals using a yacht across a river. He has 20 cows, 15 foxes and three times as many zebras as foxes. To balance the yacht to ensure a smooth sail across the river, the total number of animals in the yacht needs to be 100. If he decides to add sheep to the yacht to make the yacht sail-worthy, how many sheep did he add to the yacht?
Can you help?