Consider this example:
Many persons attended a conference. Doughnuts were served.
You were cleaning up aferwards and there were 5 doughnuts left on a plate and 3 unopened boxes of doughnuts, 12 to a box.
How many doughnuts were left?
3 boxes and 5 more: 3 boxes + 5: 3(12) + 5 = 36 + 5 = 41 doughnuts.
The multiplication tells you how many were in the boxes (there were 36 doughnuts in the 3 boxes); you shouldn't add the 5 to the number in each box, because there weren't 17 in each box!
The multiplication tells you how many individual elements there were in all the sets; the addition tells you how many individual items were left over.
So, if you have a problem like: 7 + 5(10), you have 7 individual items and 5 sets of 10 items -- there are 5 x 10 or 50 items in the sets plus the original 7, for 50 + 7 = 57.