Solve 0.15x + 0.15x = 87870 for x.

Step-by-step solution

1. 1

   Combine the like terms. Both terms are multiples of $x$ with the same coefficient, so

   $0.15x+0.15x=0.30x$

   Note it is $0.30x$, not $0.15x$ and not $0.0225x$ — adding, not multiplying.

2. 2

   Isolate $x$ by dividing.

   $0.30x=87870\;\Longrightarrow\;x=\frac{87870}{0.30}$

3. 3

   Clear the decimal. Multiply numerator and denominator by $10$:

   $\frac{87870}{0.3}=\frac{878700}{3}$

   Dividing by $0.3$ is the same as multiplying by $\frac{10}{3}$, which is why the answer is well over three times the right-hand side.

4. 4

   Divide, and see why it comes out whole. The digit sum of $878700$ is $8+7+8+7+0+0=30$, a multiple of $3$, so the division is exact:

   $x=\frac{878700}{3}=292900$

5. 5

   Substitute back. $0.15\times 292900=43935$, and $43935+43935=87870$, exactly the right-hand side. So $x=292900$ with no rounding at all.