We have two types of textbooks. Let's use "x" to represent the number of math textbooks and "y" to represent the number of history textbooks.
Altogether, the bookseller sold 240 math and history textbooks. So:
x + y = 240
Now, the number of history textbooks sold was 72 less than the number of math books sold. So:
y = x - 72
Now we can substitute for y in the equation and solve:
x + y = 240
x + (x - 72) = 240
2x - 72 = 240
2x = 312
x = 156
The bookseller sold 156 math textbooks.
Since y = x - 72, he must have sold 84 history textbooks.
Let's check our answers:
x + y = 240
156 + 84 = 240
That is true, so we did get the correct answers.
