It's common for English learners to struggle with how to pronounce large numbers.
Practice your skills in pronouncing large numbers here.
Remember to enunciate clearly the difference between '15' and '50.' This is a challenge even for native English speakers.
An easy hack for marking the difference between the two is syllable stress + the strength of the 't' sound:
15 --> second syllable stress + strong "t" --> fif TEEN
50 --> first syllable stress + gentle "t" --> FIF dee
hack (noun)(slang) = a clever solution to a tricky problem