Final answer:
The correct equation to use to find the number of teams t in the contest is 15t = 90, where t is the number of teams.
Step-by-step explanation:
To find the number of teams (t) Darrell is expecting in the balloon-launcher contest, we need an equation that relates the total number of water balloons given out to the number of balloons each team receives. Since each team gets a bucket of 15 water balloons and the total number given out is 90, we can express this situation with the equation 15t = 90, where t represents the number of teams.