Number of biology textbooks sold: 257 textbooks
Number of psychology textbooks sold: 181 textbooks
Let the total number of biology textbook sold be "a"
Let the total number of psychology textbook sold be "b"
If a textbook store sold a combined total of 438 biology and psychology textbooks in a week, then:
a + b = 438 ................... 1
If the number of psychology textbooks sold was 76 less than the number of biology textbooks sold, then;
b = a - 76 ...................... 2
Substitute equation 2 into 1
a + (a-76) = 438
a + a - 76 = 438
2a = 438 + 76
2a = 514
a = 514/2
a = 257 biology textbooks
From equation 2;
b = a - 76
b = 257 - 76
b = 181 psychology textbooks
Number of biology textbooks sold: 257 textbooks
Number of psychology textbooks sold: 181 textbooks
