Question 25093
Solve:
{{{1/p + 1/q = 1/f}}} Add the fractions on the left sides. Common denominator is pq.
1) {{{(p+q)/pq = 1/f}}} Multiply both sides by pq.
2) {{{(p+q) = pq/f}}} Multiply both sides by f.
3) {{{f(p+q) = pq}}} Finally, divide both sides by (p+q)
4) {{{f = pq/(p+q)}}}

Notice that you could have just inverted equation 1) to arrive at the same conclusion.