Answer:
a = 20 . . . . liters
Step-by-step explanation:
To solve this, multiply the equation by 30+a:
a = 0.4(30 +a)
a = 12 + 0.4a . . . . . eliminate parentheses
0.6a = 12 . . . . . . . . subtract 0.4a
a = 12/0.6 = 20 . . . divide by the coefficient of a
20 L of alfalfa must be added
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<em>Check</em>
20/(30+20) = 20/50 = 40/100 = 0.4 . . . . . . answer checks OK
