1440 tons of grain were brought to a silo over two days. on the second day, 80% of the amount of the grain delivered during the first day was brought in. how many tons of grain were delivered to the silo on the first day?

640 tons of grain was brought in on the second day.  

The best way to arrive at this answer is to write a question. Let x be the amount of grain that was brought in on the first day. That would make the amount of grain brought in on the second day 0.8x.

Now, we can write and solve the following equation.

x + 0.8x = 1440 
1.8x = 1440 
x = 800

800 tons of grain was brought in on the first day, so if we multiply that amount by 80% we have a total of 640 tons on the second day.