Then house D is build.
The diagram shows the five houses viewed from the side, and you can see house D in the middle of houses C and E.
The diagram also shows the distance between the houses, and you can see that there are 50 metres between houses C and D.
How many metres are there between houses D and E?
When you view the five houses from the side, they appear to be evenly spaced.
But how would they look if you viewed them from above?
Houses A, B, C and E are built in a row, but house D is not part of it.
House D appears to be built a little farther back.
Viewed from the side.
House D is right in the middle of houses C and E.
Which means that the distance between houses D and E should be the same as the distance between houses C and D.
The answer is '50'.