Answer:
![m = 17.87 kg](https://img.qammunity.org/2020/formulas/physics/high-school/cvza5kdh7fkg96zfulhmhvqiwysgvlonbr.png)
Step-by-step explanation:
As we know that tank itself has mass given as 13.4 kg
so it is given as
![m_1 = 13.4 kg](https://img.qammunity.org/2020/formulas/physics/high-school/ex0bnc4j5qa1al343vf8i9plpp7ajk47j2.png)
also the tank contains air in it and the mass of air inside the tank is also given as 4.47 kg
so it is
![m_2 = 4.47 kg](https://img.qammunity.org/2020/formulas/physics/high-school/7pneto74lzb34y618fxcp55exx9xdjtn97.png)
so total mass of the tank is the mass of the empty tank and mass of air in the tank
![m = m_1 + m_2](https://img.qammunity.org/2020/formulas/physics/high-school/2zqqgxn4yftvm9ao3uagxzh3rlud3og3q0.png)
![m = 13.4 + 4.47](https://img.qammunity.org/2020/formulas/physics/high-school/q6i56utcanppww3hd4w0fjrna8h1qhn1bw.png)
![m = 17.87 kg](https://img.qammunity.org/2020/formulas/physics/high-school/cvza5kdh7fkg96zfulhmhvqiwysgvlonbr.png)