question:

"If gas is $1.20 per gallon, how much would it cost to fill a 50-liter tank?"
using conversion factor.

1 L = 1.056 quarts.
4 quarts = 1 gallon.
Convert 50 L to gallons and multiply by 1.20/gallon.
Let me know if this is sufficient.

I think I got it. Thanks.

You're welcome! I'm glad I could help you. If you have any more questions, feel free to ask.