A + p% = A + A*p/100 = A*(1+p/100)
A - p% = A - A*p/100 = A*(1-p/100)



A - 30% = A*(1-30/100) = A*0.7

A - 30% = 20 <==> A*0.7 = 20 euros <==> A = 20/0.7 = 28.57... euros