Question 1057931
Let {{{ t }}} = time in hrs for the larger pipe
to fill the tank
{{{ t + 1.7 }}} = time in hrs for the smaller
pipe to fill the tank
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Add their rates of filling to get their 
rate filling together
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rate for smaller pipe:
[ 1 tank filled ] / [ t + 1.7 hrs ]
rate for larger pipe:
[ 1 tank filled ] / [ t hrs ]
rate for both pipes together:
[ 1 tank filled ] / [ 8.4 hrs ]
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{{{ 1/( t + 1.7 ) + 1/t = 1/8.4 }}}
Multiply both sides by {{{ t*( t+1.7 )*8.4 }}}
{{{ 8.4t + 8.4*( t + 1.7 ) = t*( t + 1.7 ) }}}
{{{ 8.4t + 8.4t + 14.28 = t^2 + 1.7t }}}
{{{ t^2 - 15.1t - 14.28 = 0 }}}
Use quadratic formula
{{{ t = (-b +- sqrt( b^2 - 4*a*c )) / (2*a) }}}
{{{ a = 1 }}}
{{{ b = -15.1 }}}
{{{ c = -14.28 }}}
This is some tough calculations. You can finish
and {{{ t }}} is time in hrs for larger pipe to fill tank
Check my math so far