Matthew has 18 sets of baseball cards. each set has 12 cards. about how many baseball cards does matthew have? the key word is about?

18 groups of 12. 18 times 12.  That equals 216. 216 can round to 200, 215, or 220.

Yes The key word is about and the word about wants us to estimate and round

So 18 is rounded to 20
And 12 is rounded to 10

Now multiply 20•10 which is about 200 cards 
You're multiplying 20 and 10 because your looking for the total amount in all 18 (20 because rounded) packs of baseball cards
And multiplying the 12 (10) tells you about how many card are in the packs altogether

so the actual answer would be 216 base ball cards and thats not rounded
So 200 is the rounded (estimated) answer and 216 is the actual answer for the number of base ball cards Matthew has