Which number is common factor 67 and 100

Kelly is a salesperson at a shoe store, where she must sell a pre-set number of pairs of shoes each month. at the end of each work day the number of pairs of shoes that she has left to sell that month is given by the equation s=300-15x , where s is the number of pair of shoes kelly still needs to sell and x is the number of days she has worked that month. what is the meaning of the number 300 in this equation

Aadc is formed by reflecting aabc across line segment ac, as shown in the figure. if the length of ac is 4 units, the area of aadc is square units.

Isaac wants the equation below to have no solution when the missing number is placed in the box. which number should he place in the box?