It takes 15 days to build a highway with a length of 500 kilometers. The ratio of the length of the repaired length to the length of the unrepaired length is 1:4. How many days will it take to complete the road according to this work efficiency?
修一条长500千米的高速公路，修了15天后，已修的长度和未修的长度之比是1：4，按这样的工效修完公路，一共要用多少天？
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<gadget id="calculator">60 + 15</gadget>
<output>75</output>

<result>75</result>