x-numarul initial
15% din x= 15/100 * x= 3/20 * x= 3x/20 -prima data
x-3x/20= (20x-3x)/20= 17x/20 -restul
25% din 17x/20= 25/100 * 17x/20= 1/4 * 17x/20= 17x/80 -a doua oara
17x/20- 17x/80= (68x-17x)/80= 51x/80 -noul rest
51x/80=153
x=153 *80/51
x=3*80 = 240 piese