x-pretul initial
15% din x= 15/100 *x= 3/20* x= 3x/20- scumpirea
x+ 3x/20= (20x+3x)/20= 23x/20-noul pret
5% din 23x/20= 5/100 *23x/20= 1/20 * 23x/20= 23x/400-noua scumpire
23x/20 + 23x/400= (460x+23x)/400= 483x/400
483x/400= 10.143
x=400*10.143/483
x=400*21
x=8400