The formula we can use here is:
g = G m / r^2
where g is gravity, G is gravitational constant, m is mass, and r is radius
Since G is constant, therefore we can equate two situations:
g1 m1 / r1^2 = g2 m2 / r2^2
(10 m/s^2) r1^2 / m1 = g2 * (1/2 r1)^2 / (1/8 m1)
g2 = 5 m/s^2