There is a row of 15 trees in a garden. Starting from the first tree, a gardener marked every second tree, then, on his way back, he marked the first tree, and then every third tree. How many trees are left without a mark?

Plz determine whether the polynomial is a difference of squares and if it is, factor it. y2 – 196 is a difference of squares: (y + 14)2 is a difference of squares: (y – 14)2 is a difference of squares: (y + 14)(y – 14) is not a difference of squares