Question
In a zoo, tiger and ostrich were counted. If heads were
counted, there are 400 and if legs are counted there were 820. How many ostrich are there?Solution
By allegation rule Average number of legs per head = `520/400` = `31/20` 
Tiger : Ostrich = 1 : 39 Number of ostrich = `400/40xx` 39 = 390
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