Xiao Qiang's work efficiency per minute is115, and Xiao Li's is 1/20.

Xiao Qiang worked alone for 8 minutes and finished 8/ 15.

The remaining 7/ 15 work is done by Xiao Qiang and Xiao Li, and the working efficiency per minute is115+1/20 = 7/60.

7/ 15 divided by 7/60=4, Xiao Qiang and Xiaoli cleaned together for 4 minutes.

Xiao Qiang needs 8+4= 12 minutes.