Answer:
This problem can be solved by using a system of linear equations. Let's denote:
P = the number of psychology textbooks sold, and
B = the number of biology textbooks sold.
We know from the problem statement that:
1) P + B = 220 (the total number of textbooks sold is 220)
2) B = P - 64 (the number of biology textbooks sold is 64 less than the number of psychology textbooks)
We can substitute equation 2 into equation 1 to find the value of P:
P + (P - 64) = 220
2P - 64 = 220
2P = 220 + 64
2P = 284
P = 284 / 2
P = 142
So, 142 psychology textbooks were sold.
We can then substitute P = 142 into equation 2 to find B:
B = 142 - 64
B = 78
Therefore, 142 psychology textbooks and 78 biology textbooks were sold.