The ratio of sand to cement is 1:4, which means that for every 4 units of cement, there are 1 unit of sand. If we let x represent the number of bags of cement, then the number of bags of sand is 4x.
We are given that there are 100 bags of sand, so we can set up the equation:
4x = 100
x = 100 / 4
x = 25
Therefore, 25 bags of cement are needed to mix with 100 bags of sand.
Hope this helps