A pendulum bob of mass 50 g hangs from the ceiling of an elevator....

Mass of the bob, $m = 50 \text{ g} = 0.05 \text{ kg}$

The elevator is travelling downward with a deceleration, which means its acceleration is directed upwards. Let this acceleration be $a = 1.2 \text{ m/s}^2$.

Using Newton's second law with respect to the ground frame, the forces acting on the bob are the tension $T$ upwards and the weight $mg$ downwards.

The equation of motion is:

$T - mg = ma$

$T = m(g + a)$

Taking $g = 9.8 \text{ m/s}^2$, we get:

$T = 0.05 \times (9.8 + 1.2)$

$T = 0.05 \times 11.0$

$T = 0.55 \text{ N}$