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Total amount = $520 
Sharing Ratio = 2 : 4 : 7 

First Student Ratio : 2 
Second Student Ratio : 4 
Third Student Ratio : 7 
Total Quantities of ratio = 2 + 4 + 7 = 13
Formula applied : 
         Share =  Ratio of Student /  Total quantities of ratio × Total amount 
First Student share : 
 = 2/13 × 520 
 = $80
Second student share:
 = 4/13 × 520 
 = $160
Third Student share : 
 = 7/13 × 520
 = $280 . 
Proof : 
Total share = $520 
 ( 80 + 160 + 280 )= $520 
 $520 = $520