Round each number to make it easier to multiply.
So in the case of the wedding, 19 to 20 and 8 to 10. 20 * 10 = 200. 200 forks can be your estimate.
As for your shopping problem, round $3.99 to $4 and $0.89 up to $1. $4 * 8 = $32 and $1 8 8 = $4. So my estimate would be they spent about $36.
