Question 21
Positive integers $a,b,c,d$ satisfy $a\le b\le c\le d$. How many quadruples $(a,b,c,d)$ satisfy $ab=cd=a+b+c+d-3$?
Answer format: an integer without units. Leading zeros are optional; the source uses four digit boxes.
Answer format: an integer without units. Leading zeros are optional; the source uses four digit boxes.