How to calculate the total resistance fo a parallel circuit in series
The way you do it is correct, but the numbers don't check out:

\$\frac{1}{100 \Omega}\$ is 0.01 S and not 0.1. \$\frac{1}{250 \Omega}\$ is 0.004 S and not 0.04. So you end up with 0.0104 S, of which the inverse is 71.429 \$\Omega\$ (which has way to many digits).