Question 133571
Add the rates of plowing with the 2 tractors
(1 field / 3 hrs) + (1 field / 5 hrs) = (x fraction of field / 1 hr)
{{{1/3 + 1/5 = x/1}}}
multiply both sides by {{{3*5}}}
{{{5 + 3 = 15x}}}
{{{x = 8/15}}}
This is the fraction of a field they plow
together before school. There is {{{1 - (8/15) = 7/15}}}
left for father to finish alone
Let {{{t}}} = hours for father to plow {{{7/15}}} of a field
{{{1/3 = (7/15)/t}}}
This is just (father's rate = father's rate)
multiply both sides by {{{t}}}
{{{(1/3)*t = 7/15}}}
multiply both sides by {{{15}}}
{{{5t = 7}}}
{{{t = 7/5}}} hrs
This is {{{1}}} and {{{2/5}}} hrs or
1 hours and 24 minutes for father to finish plowing the field