1.6 g
First, calculate the change in temperature.
30.5 - 23.2 = 7.3 K
Now we have 3 pieces of data, the units involved are K, J/(g K), and J. We wish to create a result with the units being g. Let's start with the specific heat.
0.90 J/(g K)
Now let's cancel out the K by multiplying by our data with K
0.90 J/(g K) * 7.3 K = 6.57 J/g
Now we want to cancel out the J and get g on top. We can do that by dividing the J value we have by the calculated value above. So
10.3 J / 6.57 J/g = 1.567732116 g
Rounding to 2 significant figures gives us a mass of 1.6 g
Remember, for most problems, simply look at the units you have and what you desire for the units in the final result. By paying attention to this, the math operations you need are pretty automatic.
