Sand is $3 per bag and pea gravel is $4 per bag.
This is an example of solving two equations in two unknowns. Each purchase must be "translated" into an equation:
If we use X for the sand and Y for the pea gravel, the two purchases can be written as
The second equation simplifies (divide each side by 2) into
so, sand is $3 per bag.
Now, place this value into the other equation:
Pea gravel is $4 per bag.