Complex sentence: The experiment's participants were given the following problem: you have 3 water jars, each with the capacity to hold a different, fixed amount of water; figure out how to measure a certain amount of water from these jars.
Split into simpler sentences: The experiment's participants were given the following problem: you have 3 water jars, each with the capacity to hold a different, fixed amount of water. The subjects had to figure out how to measure a certain amount of water from these jars.