At the time set for the hearing of the petition, the board of county commissioners shall first ascertain and determine whether notice has been given of the time of hearing as is required by this act and if it shall determine that such notice has been given to make a declaration and finding of that fact and cause the same to be entered upon its records and thereupon to hear all persons in favor of or opposed to granting the prayer of the petition and all other evidence that it may desire to hear for the purpose of ascertaining, when the petition is by taxpayers, whether such petition contains the proper number of signers possessing the qualifications prescribed by this act, and whether by taxpayer or by the directors of the drainage district whether the statements in the petition are true; and if upon such hearing it is found that such petition is in conformity to the requirements of this act, and that the allegations thereof are true, then such board of county commissioners shall make a finding and decision to that effect, and shall thereupon immediately declare the territory described in the petition to be annexed to and included in the original drainage district. No territory within any incorporated city located in a county having an assessed tangible valuation of more than $150,000,000 shall be included within any drainage district unless the consent of the governing body of such city to the inclusion of such territory shall first be obtained.