Question 410339
Add the rates of filling:
(1 tank filled/3 min) + (1 tank filled/7 min) = (1 tank filled / x min)
{{{1/3 +  1/7 = 1/x}}}
Multiply both sides by {{{21x}}
{{{7x + 3x = 21}}}
{{{10x = 21}}}
{{{x = 2.1}}}
both pipes together take 2.1 minutes
check answer:
{{{.3333 + .14286 = .47619}}}
{{{1/2.1 = .47619}}}
OK