Question 103059
Rate x Time = Distance
1. {{{R*T=D}}}
D = 20x+40 miles
R = 20 mph
From 1,
1. {{{R*T=D}}}
{{{cross(R)*T/cross(R)=D/R}}} Multiplicative inverse of R. 
{{{T=D/R}}}
{{{T=(20x+40)/20}}}Substitute. 
{{{T=20x/20+40/20}}}Distributive property.
{{{T=x+2}}}Simplify. Units are hours since {{{miles/(miles/hr)=hr}}}