The relation of heat (Q), mass (m), and change of temperature (ΔT) is given by the formula:
Q = m*Cs*ΔT, where Cs is the specific heat of the material.
Then, given than you know Q, m and ΔT, you can solv for Cs:
Cs = Q / (m*ΔT)
Cs = 89.6 J / [20 grams * (40.0°C - 30.0°C)] = 0.448 J / g * °C.
