CSS flexbox step 20

Create a media query for screens smaller than 800px in width. In that media query, create a #gallery img rule and set the width property to 50% . This will convert your gallery to a two-column layout.
my code:

@media (max-width: 800px)
@media (width: 50%)

someone please tell how should I write the code for this step?

You should have something like this;
@media (max-width: 800px) {
#gallery img {
width: 50%;

You are missing few curly brackets that is all chief. I hopoe this help solve the problem.