## Algebra 1

$12.7$ 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