Question 936779
{{{system(R=P+5,R=S+7,R+P+S=21)}}}

Substitute (P+5) for R in the 2nd and 3rd.
Solve the 2nd for S
Substitute that for S in the 3rd
Substitute back to find P and R.

Solutions: P=6, R=11, S=4

Edwin</pre>