Let Richard's speed be R
Let Gracie's speed be G
We use these formulas:
R=G+7
And the distance speed time relationship of
d=vt
where v is speed. Then we write an equation:
d=vt becomes
292=Rt. (Richard drives 292km in a time t at speed R)
The other equation is for Gracie:
264=Gt
Now use R=G+7 in the first equation to get
292=(G+7)t
Solving both of these equations for t by dividing by the speeds gives
264/G=t
292/(G+7)=t
Now, because they drove these distances in the same time, t, we can set these two equations equal because the t's that they are both equal to are the same value.
264/G=292/(G+7)
To get the G's on one side of the equation we first cross multiply to get
292G=264(G+7)
Multiply the 264 through the parentheses
292G=264G + 1848
Subtract 264G from both sides
28G=1848
Divide by 28 to get G, Gracie's speed
G=66.
Now recall
R=G+7
R=73 km/hr
