To start, you need to know the conversion rate to get the answer. You can find this in the first half of the word, the root DECI or DEC means 10, you'll find this everywhere, such as in a word like Decade (or you can look it up).
This tells us that one meter is equal to 10 decimeters.
So, using the formula M = 1/10 • d
M = meters
d = decimeters
We can replace d with 50 and then we will get our answer.
M = 1/10 • 50
M = 50/10
M = 5
Therefore there are 5 meters in 50 decimeters.