A block of ice of mass 120g at temperature 0 ° C is put in 300g of...

According to the principle of calorimetry, heat lost by hot body is equal to the heat gained by cold body.

The heat released by water

$∆ Q = ms ∆ T = 0 . 3 \times 4200 \times 25 = 31500J$

let $mkg$ ice melts then,

$∆ Q = mL = m \times 3 . 5 \times {10}^{5} = 31500J$

$⇒ m = \dfrac{31500 \times {10}^{- 5}}{3 . 5} = 9000 \times {10}^{- 5}$

$m = 0 . 09kg = 90g$

$⇒ x = 90$