Chapter 11 - Rational Expressions and Functions - 11-5 Solving Rational Equations - Practice and Problem-Solving Exercises - Page 683: 24

$12.7$ minutes

Let both david and allie work together in $T$ minutes (Fraction work done in a minute by david)+ (Fraction work done in a minute by allie)T=1 $\implies (\dfrac{1}{20}+\dfrac{1}{35})T=1$ $\implies (\dfrac{35+20}{700})T=1$ $T=\frac{700}{55}$ or, $T=12.7$ minutes