The amount of heat (*H*) produced is given by the joule's law of heating as*H*= Vlt

Where,

Current, *I* = 5 A

Time, *t* = 30 s

Voltage, *V* = Current x Resistance = 5 x 20 = 100 *V*

*H*= 100 x 5 x 30 = 1.5 x 10^{4} J.

Therefore, the amount of heat developed in the electric iron is 1.5 x 10^{4} J.