How many paying customers are needed to made a profit? thank you I think it is 19 but I am not sure

Profit will be positive when

0.4x

^{2}-7.2x≥0;The minimum number of customers is determined by considering the case, when 0.4x

^{2}-7.2x=0This expression is equivalent to:

0.4x-7.2=0 (x>0, number of customers can not be zero, therefore we can divide both side by x)

From this we obtain:

x=7.2/0.4=18

So 18 customers give zero profit, therefore any number greater than 18 will suffice.