Tom sells baseball cards at 10 for 35 cents. is that a better deal than 12 for 40 cents?

Let's calculate how much he spends per each individual card using proportions:

In the first case he spends 35 cents for 10 cards:
35 cents / 10 cards  =  x cents / 1 card
35/10 = 3.5 cents per card!

In the second case he spends 40 cents for 12 cards:
40 cents / 12 cards  =  x cents / 1 card
40/12 = 3.333 cents per card!

Now compare the two :)