A phone card costs $5$ euros. This can be used to call someone for $60$ minutes within the zone. For a house phone one pays $14,25$ euros as a bundle and $0,15$ euros for every $12$ minutes. How long can one call per month so that the card is cheaper than with the house phone?
I already know it's at most 180 minutes with the card because $0,15 \cdot 5 = 0,75$ (you multiply by $5$ because $60 \div 12$ minutes $=5$. And $14,25 + 0,75 = 15$ so you can buy $3$ phone cards with it resulting in $180$ minutes. I don't know how to write it in inequality.
Let's say that you speak $x$ minutes. Then, the total price for the house phone is $$ h(x) = 14.25 + 0.15 \lceil \frac{x}{12} \rceil $$ Phone cards, on the other hand, have a total cost of $$ c(x) = 5 \lceil \frac{x}{60} \rceil $$ Here $\lceil \ldots \rceil $ is the ceiling function. So for example $\lceil \frac{x}{12} \rceil$ is how many "units" of 12 minutes we need to buy.
Here are the graphs of the two functions: https://www.desmos.com/calculator/dh9gwbgokp
Can you continue from here?