In this blog [Tutorial] A way to Practice Competitive Programmig by E869120 writed:

Sometimes, especially in Div1+Div2 round, some problems need mathematical concepts or thinking. Since there are a lot of problems which uses them (and also light-implementation!) in TopCoder, you should solve TopCoder problems.

So maybe TopCoder Div1Easy good place to improve math. (I haven't tried it yet)