You can put this solution on YOUR website! The short way:
Since 10 days is 2 times 5 days, the rabbits would eat
12.5 lettuce the first five days, and
another 12.5 lettuce the next five days,
for a total of lettuce.
The general way to do it:
In one day, those same rabbits would eat lettuce (the 12.5 they ate in 5 days divided by the 5 days)
In days they would eat times as much as in one day, so it would be lettuce.