Free time - Answer #4:
`x + x xx 18 + x xx 20 + ...+ x xx 48 = 23805`
`=> x + (x xx 18 + x xx 20 + ...+ x xx 48) = 23805`
`=> x + x xx (18 + 20 + ...+ 48) = 23805`
`=> x + x xx {(18+48) xx [(48 - 18) : 2 + 1] : 2} = 23805`
`=> x xx 1+ x xx (66 xx 16 : 2) = 23805`
`=> x xx (528 + 1) = 23805`
`=> x xx 529 = 23805`
`=> x = 45`
Vậy `x = 45`