$30$\%+$40$\%=$70$\%,  $50 \times 70$\%=$35$  After two months, $35$ miles of road can be built. 
A construction team is going to build a road of $50$ miles. The plan is that it finishes $30$\% of the road in the first month, $40$\% of the road in the second month and $30$\% of the road in the third month. How many miles can be built after two months? 
- $$15$$ 
- $$20$$ 
- $$30$$ 
- $$35$$ 