In a restaurant kitchen, a chef had 40 $$l$$ of cooking oil. <br>He used 15 $$l$$ of the cooking oil on Saturday and another 18 $$l$$ of cooking oil on Sunday.<hr> He had <input min="0" type="number" id ="imp2pg224q7_1" class="lineinput" placeholder="?" /> $$l$$ of cooking oil left.