A liquid of volumetric thermal expansion coefficient  $200\times {10}^{–6}/°C$ and bulk modulus = $1.2\times {10}^{9}Pa$  is filled in a spherical tank of negligible heat expansion coefficient. Its radius is 25 cm and wall thickness is 2 mm. When the temperature of the liquid is raised by $20 °C$, find the tensile stress developed (in MPa) in the wall of the tank?