Step 1: Make all the exponents the same

To add or subtract numbers in scientific notation we must make all the exponents the same:

\[

\begin{aligned}

1,99 \times 10^{-26} &=0,199 \times 10^{-25} \\

\text { and } 1,67 \times 10^{-27} &=0,0167 \times 10^{-25}

\end{aligned}

\]

Step 2: Carry out the addition and subtraction

Now that the exponents are the same we can simply add or subtract the \(N\) part of each number:

\[

0,199+0,0167-2,79=-2,5743

\]

Step 3: Write the final answer

To get the final answer we put the common exponent back:

\[

-2,5743 \times 10^{-25}

\]