Martha and mary had 375 jelly beans in all. after mary ate 24 jelly beans and martha ate 1/7 of her jelly beans, they each had the same number of jelly beans left. how many jelly beans did each girl have at first?

Start by setting up two equations

Let x = number of beans with Martha
Let y = number of beans with Mary

Since total beans is 375, 
x + y = 375

Mary ate 24 beans, after this she must be having x - 24 beans
Similarly after eating 1/7y beans, mary must be having y - y/7 beans
Setting them equal to each other
x - 24 = y - y/7

Solving both equations gives x = 186, y = 189