How to get these two divs side-by-side?

Question

I have two divs that are not nested, one below the other. They are both within one parent div, and this parent div repeats itself. So essentially:

Answer 1

Using flexbox it is super simple!

#parent_div_1, #parent_div_2, #parent_div_3 {
    display: flex;
}

Fiddle example