7 cans --------------------- 3 days
x cans ---------------------- 3 + d days
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We will make a proportion:
7 : x = 3 : ( 3 + d )
or:
7 / x = 3 / ( 3 + d ) / Then we will cross multiply.
7 * ( 3 + d ) = 3 x
21 + 7 d = 3 x
x = ( 21 + 7 d ) : 3
x = 7 + 7d/3 ( and this is your answer ).
