Tension T provides the centripetal force, $T = mv^2/r$. Kinetic energy K.E. = (1/2)mv². From this, $mv^2 = 2 \times K.E.$. Substituting this into the tension formula, T = (2 × K.E.) / r. Given K.E. = 20 J and r = 50 cm = 0.5 m, T = (2 × 20) / 0.5 = 80 N.