Question 387239
7u+3r=724
3u+7r=1276 
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21u + 9r = 2172 eqn 1 times 3
21u +49r = 8932 eqn 2 times 7
---------------- Subtract
-40r = -6760
r = 169
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7u + 3*169 = 724
u = 31