Because the elongation of the spring is proportional to the tensile force, Xiao Ming lengthened the spring by 3cm and Xiao Fang lengthened the spring by 4.5cm, so the latter is 1.5 times of the former. Then the pulling force used by Xiao Ming should be 1.5 times, so the pulling force used by Xiao Fang is = 5n×1.5 = 7.5n. 。

So the answer is: 7.5.