How to select the first :nth-child(1)?

Question

css

Cho did not fully understand the principle of nth.
please explain with this example.
Only "10" needs to be yellow.
Thank you!
welovepavel.esy.es/2

.s_tizers .row div i:nth-child(1){
    color: yellow;
}

<section class="s_tizers">
        <div class="container">
            <div class="row">                
               
               
                <div class="col-md-3">
                    <div class="tiz_img">
                   <i class="my">10</i>
               </div>
               <div class="tiz_desc">
                  <span>Более 10 лет</span> 
                  <strong>На рынке</strong>
               </div>
                </div>
                <div class="col-md-3">
                    <div class="tiz_img">
                   <i class="my my_d"> </i>
               </div>
               <div class="tiz_desc">
                  <span>самые современные</span> 
                  <strong>методики</strong>
               </div>
                </div>
                <div class="col-md-3">
                    <div class="tiz_img">
                   <i class="my my_e"> </i>
               </div>
               <div class="tiz_desc">
                  <span>квалифицированные</span> 
                  <strong>преподаватели</strong>
               </div>
                </div>
                <div class="col-md-3">
                    <div class="tiz_img">
                   <i class="my my_f"></i>
               </div>
               <div class="tiz_desc">
                  <span>гарантированное получение</span> 
                  <strong>высоких баллов</strong>
               </div>
                </div>
            </div>
        </div>
        
        
    </section>

Answer 1

.s_tizers .row .col-md-3:first-child i {
    color: yellow;
}

Answer 2

.s_tizers .row + div i{
color: yellow;
}

Answer 3

Learn the mathematical part (Dmitry Potapenko ©)
htmlbook.ru/css/nth-of-type