Let x=number of free throws that Caron has made so far.
Out of these, 0.875x were successful. If we add 13 to this we get 13+0.875x as the total number of successful throws in x+20 free throws, so the fraction is (13+0.875x)/(x+20)=0.86.
(0.875-0.86)x=17.2-13=4.2. And 0.015x=4.2, so x=4.2/0.015=4200/15=280.
So far, then, Caron has had 280 free throws, 245 of which were successful. [(245+13)÷(280+20)=258/300=0.86.]