Помогите решить,пожалуйста!! please help me to solve this!!

\displaystyle \left(\frac{1}{5a}+\frac{1}{7a}\right)\frac{a^2}{4} =\left(\frac{1}{5}+\frac{1}{7}\right)\frac{a}{4} = \frac{12a}{35\cdot4} = \frac{3a}{35} = \frac{3\cdot7.7}{35} = \frac{3\cdot1.1}{5} = 1.1\cdot0.6 =\\\\= 0.66