Question 907669
.20*600=600/5=120 kilos of tin  we want .25 or 1/4 or 25% 
.40*600=240 kilos of iron we want 50% or .5 or 1/2

(1) {{{ ( a + 120 ) / ( a + b + 600 ) = .25 }}}
(2) {{{ ( b + 240 ) / ( a + b + 600 ) = .50 }}}

(a + 120 ) / ( a + b + 600 ) = .25,
( b + 240 ) / ( a + b + 600 ) = .50 
a=120 tin
b=240 iron
240/960=1/4
480/960=1/2