25

fie ab.cd suma initiala
fara cei 5 bani  vom avea:

ab.ef (unde e poate fi sau nu egal cu c ... nu are importanta :D)

=>
ab.ef = 2 ef.ab
10a+b+0.1e+0.01f=20e+2f+0.2a+0.02b
10(a-2e)+(b-2f)+0.1(e-2a)+0.01(f-2b)=0 <=> a=2f & b=2f & e=2a & f=2b <=> a=f=b=c=0 => dobanda = 00,05 .. banca romeansca :))

sau:
9.8a+0.98b=19.9e+1.99f
98(0.1a+0.01b)=199(0.1e+0.01f)
cum 199 e nr prim so cmmmc = 199x98 => 0.1a+0.01b=199 & 0.1e+0.01f=98 si cum a,b,c,d ~{0,1,2,...,9} =>singura sol a=b=c=d=0 :0 ==> aceasi banca romaneasca