Let x=mass of 10% titanium and y=mass of 20% titanium.
x+y=100 (total mass) so y=100-x
0.10x+0.20y=0.16*100=16 (quantity of titanium)
Substitute for y:
0.10x+0.20(100-x)=16
0.10x+20-0.20x=16
4=0.10x so x=40g and y=100-40=60g.
40g of 10% titanium mixed with 60g of 20% titanium will make 16% of alloy.