m1 = F/a1 ; and
m2 = F/a2 . Here, a1 = 10 m s-2;
a2 = 20 m s-2 and F = 5 N.
Thus, m1 = 5 N/10 m s-2 = 0.50 kg; and
m2 = 5 N/20 m s-2 = 0.25 kg.
If the two masses were tied together,
the total mass, m would be
m = 0.50 kg + 0.25 kg = 0.75 kg.
The acceleration, a produced in the combined mass by the 5 N force would
be, a = F/m = 5 N / 0.75 kg = 6.67 m s-2