Hi Elena,
Some numbers do sound very similar to each other.
You want to practice slowly, saying the number very slowly until it sounds natural. With the number 15, it should sound like:
Fi-ffff, tea (like "afternoon tea") - eeeennnn
for 50, it should sound like
Fi-ffff, tea (like "afternoon tea")
This is how they sound, not how they are spelled. The difference between 15 and 50 is the ending. With 15 you have the "eeeeennnnn" but you do NOT have it with the 50.
I hope that helps.
- Adam