Answer
a. Let the mass of the bus be M.
The net force on the bus was Ma.
The new acceleration is found using Newton's second law. The net force is the same, but the mass has changed:
$$a = \frac{same \,force}{new \,mass} $$
$$a= \frac{Ma}{M+ M/5} = \frac {Ma}{\frac{6M}{5}} = \frac{5a}{6}.$$
b. Calculate the new acceleration, using the result just derived:
$$a= (5a/6) = (5/6)(1.2 \frac{m}{s^{2}}) = 1.0\frac{m}{s^{2}} $$.
