Chapter 11 - Rational Expressions and Functions - 11-4 Adding and Subtracting Rational Expressions - Practice and Problem-Solving Exercises - Page 676: 33

a) $1/r + 1.428/r = t$ b) $2.428/r = t$ c) .809 mph

a) alone: distance = rate * time $1 = r*t$ $1/r = t$ with grandfather: distance = rate * time $1 = .7*r*t$ $1/.7r = t$ $1.428/r=t$ $1/r + 1.428/r = t$ c) $2.428/r = t$ $2.428/3 = t$ $.809 mph$