a)

$15,75-\left(90-84,125\right)+1,09=$  

$=15,75-\left(90-84-0,125\right)+1,09=$  

$=15,75-\left(6-0,125\right)+1,09=$  

$=15,75-5,875+1,09=$  

$=9,875+1,09=10,965$      

 

b)

$\left(120,9-38,46\right)+\left(300,01-96,4\right)=$  

$=\left(120+0,9-\left(40-1,54\right)\right)+\left(300+0,01-\left(100-3,6\right)\right)=$  

$=120-40+0,9+1,54+300-100+0,01+3,6=$  

$=\underset{=280}{\underbrace{120+300-40-100}}+\underset{=6,05}{\underbrace{0,9+0,01+3,6+1,54}}=$  

$=280+6,05=286,05$