Maggie bought 3 dozen muffins from a baker for $48. find the costs of 1 dozen muffins and 9 dozen muffins.

We will say that d = 1 dozen. and y = cost

we know that 3 dozen is 48. so
3d = 48.

to get the cost of 1d, we have to get the 3 to change into a 1. the only way to do this is to divide 3 by 3. if we divided the left side by 3 we must also do the right so:
3d = 48
1d = 16

so 1 dozen = 16

to change 3d into 9d we have to multiply by 3. we again have to do this to both sides. so:
3d = 48
9d = 144