Question 1027961
Workrate for A, {{{1/(b-10)}}}

Workrate for B, {{{1/b}}}

Rate of A and B together, {{{1/12}}}


Meaning {{{1/12=1/(b-10)+1/b}}}.  Solve for b.


That should become {{{b^2-34b+120=0}}}.
Two numbers will occur for b, but the realistic result is {{{highlight(b=30)}}}.