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Let x = small soda cost/price), in dollars.
Then the medium soda costs (x+0.7);
the large soda costs 2x;
the jumbo size costs 2*(x+0.7).
x + (x+0.7) + 2x + 2*(x+0.7) = 9.30
x + x + 0.7 + 2x + 2x + 1.4 = 9.30
6x + 2.1 = 9.30
6x = 9.30 - 2.1 = 7.2 ====> x = = 1.2.
Thus small soda costs 1.2 dollars.
Based on it, calculate the cost of other sodas on your own.
The lesson to learn from the solution is THIS:
This problem is for one unknown. It is not for 4 unknowns and not for using systems of equations.