Joe, Bob and Dan worked in the ratio of 1:2:4 hours, respectively. How

ok, so knowing the ratio for each person, we can rewrite as x:2x:4x, where X is a factor, and the total time they worked can be written as the sum of x, 2x, and 4x. Therefore, in order to find the answer to our question, we need to find out either total hours worked, or X.
(1) tells us the total number of hours worked by Joe, Bob and Dan. Thus, we can solve for X.
49 = 7x
x = 49/7
x = 7
Bob worked 2x => he worked 7*2 = 14 h. sufficient.

(2) Dan worked 21 hours more than Joe. We can rewrite this as 4x - x = 21. From this, we can solve for X.
3x = 21
x = 7
Bob worked 7*2 = 14 h. sufficient.

Of course, for DS, we do not need to do all the calculations, just to check if we can solve for X or not.