Jacky bakes some biscuits. he keeps 3/7 of the biscuits in container a, 5/8 the remainder in container b, and the rest in container c. there are 21 more biscuits in a than c. how many biscuits does jacky bake?

 Let number of biscuits baked = x. 
(3/7)x are in container a. 
The remainder = (4/7)x, because (3/7)x + (4/7)x = (7/7)x = x. 
Now, (5/8)*(4/7)x = (5/14)x are in container b. 
The rest = (3/8)*(4/7)x, because (5/8)*(4/7)x + (3/8)*(4/7)x = (8/8)*(4/7)x = (4/7)x. 
So, (3/8)*(4/7)x = (3/14)x are in container c.