find a and b in $a67.9b In the following problem, $A$ and $B$ are single digits. 

A store sold 72 decks of cards for $\$A67.9B$. Find $A + B$.

Here is as far as I can go: The price of each deck of card should be $\frac{A67.9B}{72}$, and $72=2^3\times 3^2$. How do I find $A$ and $B$ from here?

The number of cents of the total cost must be divisible by $8$. There is only one thing $B$ can be to make that happen (note that the value of $A$ does not affect whether the total is divisible by $8$).
The number of cents of the total cost must be divisible by $9$. Once you've found $B$, there is only one possible $A$ that allows this.
