Four people on one side of the river need to cross the river in a boat that can carry a max of 180 pounds and the weights are 80 100 150 and 170 and how they get to the other side?

the two who weigh 100 both get in the boat. they sail across one gets out on the other bank. the other comes back. the one who weighs 160 gets in the boat and sails across alone and gets out then the person who weighs 100 gets in and sails back across. the other person gets in and they both sail accross together again.