text: From your question, I can write 2 equations with 2 unknowns. Let x be the mls of 3% and y the mls of 15%. We know x+y=2500 and ,03x+.15y=.1(2500)=250ml. Substitute for x, since x=2500-y to get .03(2500-y)+.15y=250. Expanding and combining yields  75-.03y+.15y=250, then .12y=175, then y=175/.12 =1458+1/3, so x=1041+2/3.