Question 971470
Average speed is always:
[ total distance traveled ] / [ time to travel that distance ]
Let {{{ d }}} = one-way distance to grandma's house
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Distance to house:
{{{ d = 20t[1] }}}
Distance going back:
{{{ d = 35t[2] }}}
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Ave speed:
{{{ 2d / ( t[1] + t[2] ) }}}
{{{ 2d / ( d/20 + d/35 ) }}}
Divide top and bottom by {{{ d }}}
{{{ 2 / ( 1/20 + 1/35 ) }}}
Multiply top and bottom by {{{ 140 }}}
{{{ 280 / ( 7 + 4 ) }}}
{{{ 280/11 = 25.4545 }}} mi/hr
Hope this makes sense