The quantity of heat needed to melt ice = mL (where m is the mass of ice and L is the specific latent heat of fusion of ice)
Q = $50\times 336=16800J$ but this is 70% of the total heat supplied so that we have,
70% total heat = 16800J
$\frac{70}{100}$ of total heat = 16800J
⇒ Total heat supplied = $\frac{16800\times 100}{70}=24000J$