Very easy. Square the first equation you get a+b+2 root"a"root "b". Equation is a+b+20=100.So a+b=80. Substitute this .By squaring this you aSquare+b square+2ab=6400. So u get a×a+b×b=6400-200=6200. You bringby either substituting for a or for b. You get quadratic equation in one variable with two solutions. And then substitute to get the value of other.