80 bookmarks total = Dorothy + Patricia = D + P = 80
Dorothy has 3/5 of the number Patricia has >> D = 3/5 * P
If D + P = 80, then D = 80 - P
We can replace the D: D = 3/5 P becomes 80 - P = 3/5 P
We want to solve for P:
80 - P = 3/5 P
add P to both sides
80 = 3/5 P + P
80 = 3/5 P + 5/5 P
80 = 8/5 P
multiply both sides by 5/8
80 * 5/8 = P
50 = P
Answer: Patricia has 50 bookmarks.
Note: Since D + P = 80 and we know P = 50 we can do this:
D + P = 80
D + 50 = 80
D = 30
Dorothy has 30 bookmarks.