X^2+10x+16=0, which number would have to be added to "complete the square"?

Ican interpret and represent the remainder in division calculations sensibly for the context (eg 27.4 in calculating average height in cm: $27.44 if it represents an amount of money) solve the following problems. 15) the wildlife park charges $25.50 dollars per person to enter their park if the park made $15096 dollars today, how many people entered the park? 16) ms adams plus 2,230 other keen runners ran a marathon: between them they ran 94817.5km. how many km did each competitor run?