Hello, kwhite!
It seems that Baby-Talk is called for . . .
A total of 40 stamp sets are bought for $28.
One set costs $0.37 each while the other costs $1.25 each.
How many of each set was purchased?
Let \(\displaystyle x\) = number of the small sets.
Since 40 sets were bought, that leaves \(\displaystyle 40 - x\) for the large sets.
The \(\displaystyle x\) small sets cost $0.37 each.
. . So their cost is: \(\displaystyle 0.37x\) dollars.
The \(\displaystyle 40-x\) large sets cost $1.25 each.
. . So their cost is: \(\displaystyle 1.25(40-x)\) dollars.
Hence, the total cost of all the sets was:
.\(\displaystyle 0.37x\,+\,1.25(40-x)\) dollars.
. . But we are told that the total cost was $28.
So there is our equation! . . . \(\displaystyle 0.37x\,+\,1.25(40-x)\:=\:28\)