Find the efficiency e of the oven. you will need to use the fact that 4.2 j of energy is required to raise the temperature of 1.0 ml of coffee by 1.0∘c .

Note that
V = 1.0 mL = 1 cm³ = 10⁻⁶ m³, the volume of water (coffee)
ρ = 1000 kg/m³, the density of water
c = 4.18 kJ/(kg-K), the specific heat of water

The mass of 1 mL of water (coffee) is
m = ρV = (1000 kg/m³)*(10⁻⁶ m³) = 10⁻³ kg

The heat required to raise the temperature by ΔT = 1.0 °C (1 K) is
Q₁ = m*c*ΔT
    = (10⁻³ kg)*(4.18 kJ/(kg-K))*(1 K) 
    = 4.18 x 10⁻³ kJ
    = 4.18 J

The actual heat used is Q₂ = 4.2J.

The efficiency is
e = Q₁/Q₂ = 4.18/4.2 = 0.995 or 99.5%
Note that this efficiency does not include the efficiency of the electrical circuitry and the magnetron.

99.5%