15. 50 kg
16a. 5 kg
16b. 3.75 kg
The formula relating force, mass, and acceleration can be solved for mass. This formula will apply to both problems. We'll use m for both "mass" and "meters". We presume you can avoid getting mixed up by understanding that meters is used in the context of acceleration: m/s².
F = ma
m = F/a . . . . . divide by a
15. m = (250 N)/(5 m/s²) = 50 kg
16a. m = (15 N)/(3 m/s²) = 5 kg
16b. m = (15 N)/(4 m/s²) = 3.75 kg
Comment on units
Especially for physics problems, I like to keep the units with the numbers. It is helpful to remember that Newtons are equivalent to kg·m/s². So, dividing Newtons by acceleration in m/s² will give mass in kg. Since you're familiar with F=ma, it's not too hard to remember that the units of force (N) are the product of the units of mass (kg) and acceleration (m/s²).
y=8 or -8