The metropolitan zoological park and museum district by a vote of the qualified voters thereof voting thereon as now or hereafter required by Section 26(b) of Article VI of the Missouri Constitution may incur an additional indebtedness for the purposes of the said subdistricts separately. The total indebtedness authorized separately by the voters for the respective subdistricts shall not exceed five percent of the taxable tangible property therein as shown by the last completed assessment for state and county purposes in said city and said county.