Solving an equation when only one variable is unknown is straightforward. It is just a matter of isolating the unknown variable on one side of the equals sign. For example, we could use the equation for output in an open economy (Y=C+I+G+NX) to solve for Investment "I' when consumption 'C' is $104.6, output 'Y' is $182.5, government spending 'G' is $48.8 and net exports 'NX' is -$5.2. All numbers are in billions. First, we start with our equation: Y=C+I+G+NX And plug in the values we are given (we will leave out the "billions" label until the end, since all numbers are expressed in the same terms). Note that adding a negative number is the same as subtracting a positive number, as on the NX term: $182.5=$104.6+1+$48.8-$5.2 Now group together the constants on the right-hand side: $182.5-$104.6+ $48.8-$5.2+1 $182.5-$148.2 +1 And get the unknown variable I on one side by itself by subtracting $148.2 from both sides: $182.5-$148.2=1 And simplify to solve for l: 1- $34.3 billion In a closed economy (no imports or exports). Aggregate Expenditure 'AE' is defined as the sum of Consumption Expenditure 'C'. Investment Expenditure T and Government Expenditure 'G' or AE-C+1+0 in equilibrium, income 'Y' is equal to AE So, the equation can be rewritten as Y=C+I+G If Y-$1265, C-$660, and 1= $325, solve for G. G-$ .......................