No, I don't think so; an equation must have an equal sign in it. Expressions, however, do not have equal signs in them. Therefore I think 15 + k is an <em>expression</em>, not an equation. Hope this helps :)
explanation: If there are 50 numbers in a hat, and 5 of them are the number 10. 50 - 5 = 45 which means 45 of the numbers are not 10. Therefore the probability of not drawing 10 would be 45/50.