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In the week that ended June 19, the U.S. commercial crude oil inventory decreased by 3.8 million barrels. Now if the inventory was 353.9 million barrels in the week that followed, what was the U.S. crude oil inventory before the decline?

A 357.7

Let x represent crude oil inventory in millions of barrels for the previous week before the decline.”
Set up an Equation. “Crude oil inventory last week experienced a decline and result in the crude oil inventory now” becomes Oil inventory last week - decline in inventory = Oil inventory this week
x − 3.8 = 353.9
Solve the Equation. To isolate an unknown value x, add both sides of the equation by 3.8.
Original equation
x − 3.8 = 353.9.
Add both sides of the equation by 3.8.
x − 3.8 + 3.8 = 353.9+ 3.8
On the left, adding by 3.8 “undoes” the effect of subtracting by 3.8 and returns x. On the right, 353.9+ 3.8 = 357.7.
x = 357.7
The previous week, US crude oil inventory was 357.7 million barrels.