Determine the time required to heat 20 kg of water from 10^ ^ ^ C...

The heat required to raise the temperature of the water is given by:

$Q = mc\Delta T$

Substituting the given values:

$Q = 20 \times 4200 \times (35 - 10)$

$Q = 20 \times 4200 \times 25 = 2100000 \text{ J}$

The useful power utilized to heat the water is $80\%$ of the rated power:

$P_{useful} = \dfrac{80}{100} \times 1000 = 800 \text{ W}$

The time $t$ required to heat the water is:

$t = \dfrac{Q}{P_{useful}} = \dfrac{2100000}{800} = 2625 \text{ s}$

Converting the time into minutes:

$t = \dfrac{2625}{60} \text{ min} = 43.75 \text{ min}$