Express the results of the following calculation to the appropriate number of significant figures: 

$\frac{\left(1.36\times {10}^{-4}\right)(0.5)}{2.6}$

Express the results of the calculation to the appropriate number of significant figures:

$0.582+324.65$

Express the results of the following calculation to the appropriate number of significant figures:

$2.64\times {10}^3+3.27\times {10}^2$

Express the results of the given calculation to the appropriate number of significant figures:

$943\times 0.00345+101$

One microgram is equal to: