Our equations are:
B/S=5/4
B+S=720
By substituting the first equation solved for S, S=B*(4/5), into the second equation, we get:
B+(B*(4/5))=720
B*(9/5)=720
B=720*(5/9)
B=400
If you wanted to solve for the small number too:
(400)+S=720
S=320
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