Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1993 based on Integers.
Find the number of four topics of integers (a,b,c,d) with 0<a<b<c<d<500 satisfy a+d=b+c and bc-ad=93.
Integers
Digits
Algebra
Answer: is 870.
AIME I, 1993, Question 4
Elementary Algebra by Hall and Knight
Let k=a+d=b+c
or, d=k-a, b=k-c,
or, (k-c)c-a(k-a)=k(c-a)-(c-a)(c+a)
=(a-c)(a+c-k)
=(c-a)(d-c)=93
(c-a)(d-c)=(1,93),(3,31),(31,3),(93,1)
solving for c
(a,b,c,d)=(c-93,c-92,c,c+1),(c-31,c-28,c,c+3),(c-1,c+92,c,c+93),(c-3,c+28,c,c+31)
taking first two solutions a<b<c<d<500
or,\(1 \leq c-93, c+1 \leq 499\)
or, \(94 \leq c \leq 498 \) gives 405 solutions
and \(1 \leq c-31, c+3 \leq 499\)
or, \(32 \leq c \leq 496\) gives 465 solutions
or, 405+465=870 solutions.
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