Explanation:
x = cost of a small box
y = cost of a large box
16x + 14y =343
13x + 23y = 470.5
now we multiply both equations with factors so that we have the same number of x in both (we could the same for y, but I simply picked x, because their original factors are closer and smaller).
and then we subtract the second equation from the first and since the result for y.
13 × (16x + 14y = 343)
16 × (13x + 23y = 470.5)
gives us
208x + 182y = 4459
- 208x + 368y = 7528
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0x - 186y = -3069
186y = 3069
y = $16.50
now let's pick e.g. the first original equation to get x :
16x + 14y = 343
16x + 14×16.5 = 343
16x + 231 = 343
16x = 112
x = $7.00
the cost of a small box is $7.00
the cost of a large box is $16.50