G0j abcde la so can tjm.thj abcde*4=edcba. Ta co' a faj la so' chan. Va a<=2.vj neu a>2 thj 4a>10.dan den' thu0g la so 6 chu so.vay a=2.suy ra e=8(vj e>=4a). Xet b. ta co' 4a=e nen 4b<10.hay b<=2.ma {4d}+3=b.nen b la so le.nen b=1.tu do' suy ra d=2 hoac d=7. neu d=2 thj 4d+3=11 thj {4c}+1=c.djeu nay ko xay ra. nen d=7.suy ra 4d+3=31.nen {4c}+3=c.djeu nay xay ra khj c le? va c chj co the =9.vay so can tjm la 21978