What I did to start with was to find the lowest number that both 12 and 10 could divide into, that number is 60.
I know 60 divided by 12 is 5 and I know that 60 divided by 10 is six.
Then I took 5 and multiplied it by 3 and got 15. I did this step because you need 5 packages of cups and each package cost 3 dollars.
Next I took 6 and multiplied it by 5 and got 30. I did this step because you need 6 packages of plates and each package cost 5 dollars.
You would then take 30 dollars and 15 dollars and add them together to get your total.
The answer is 42 dollars.