Mrs quirk buys 1 quart of milk, michael drinks 2/7quart of it. joel drinks 1/3 quart. how many quart are left

Total quantity of milk bought by Mrs Quirk = 1 quart
Quantity of milk Michael drank from it = 2/7 quart
Amount of milk drank by Joe from it = 1/3 quart.
So the total amount of milk drank by Michael and Joe = (2/7) + (1/3)
                                                                                       = (6 + 7)/21
                                                                                       = 13/21 quart
Then
Amount of milk left = 1 - (13/21) quart
                              = (21 - 13)/21
                              = 8/21 quart
So 8/21 quart of milk is left after Michael and Joe drank from the 1 quart bought by Mrs Quirk. I hope this is the answer you were looking for and the procedure of doing this problem is clear to you.

What sisters? is it in a story?