At the same time, one stone is thrown vertically upward from the ground at a speed of v0 = 5m / s,

At the same time, one stone is thrown vertically upward from the ground at a speed of v0 = 5m / s, and the second one falls from a height of h = 10 m above the ground without an initial speed. How long will it take for these stones to meet?

We will take as a reference point connected with the Earth, then the equation of motion of the 1st stone will have the form:
x1 = v0t- gt ^ 2/2 where g is the acceleration of free fall.
And for the 2nd:
x2 = h-gt ^ 2/2
The meeting condition is the equality of their coordinates: x1 = x2
v0t- gt ^ 2/2 = h-gt ^ 2/2
v0t = h
t = h / v0.
t = 10 / 0.5 = 20 s