Chapter 1 - Section 1.7 - Problem Solving: Interest, Mixture, Uniform Motion, Constant Rate Job Applications - 1.7 Asses Your Understanding - Page 143: 53

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We know that in physics terms $t=\frac{s}{v}$, so if $s$ is the running distance, then $87-s$ is our running distance and then our equation is: $\frac{s}{6}+\frac{87-s}{25}=5\\25s+6(87-s)=5\cdot6\cdot25\\25s+522-6s=750\\19s=228\\s=12$ So the running distance was $12$ miles and the cycling one was $87-12=75$ miles.