x-pretul initial
10% din x= 10/100*x= 1/10*x= x/10-marirea
x+x/10=(10x+x)/10= 11x/10-noul pret
10% din 11x/10= 10/100 *11x/10= 1/10 * 11x/10= 11x/100-noua marire
11x/10 +11x/100= (110x+11x)/100= 121x/100
121x/100=13,31
x=13,31*100/121
x=1331/121
x=11
x/10 +11x/100= (10x+11x)/100= 21x/100= 21*11/100=231/100=2,31
p% din 11=2,31
p/100*11=2,31
p/100=2,31/11
p=100*2,31/11
p=231/11
p=21%