I'm just doing this one to see if I can solve it without "cheating" and looking at other's answers.....!!!
Let N be the total number of stamps....so we have
C + D = 49/70N (1)
B + D = 60/70N (2)
B + C = 620
Adding (1) and(2), we have
620 + 2D = (109/70)N → D = (109/140)N - 310
So
B + C + D = N
620 + (109/140)N - 310 = N
310 = (31/140)N → N = 1400
So D has (109/140)(1400) - 310 = 780
And B has
780 + = (6/7)(1400) → D = 1200 - 780 = 420
So D - B = 780 - 420 = 360 more
