a) x = 13,104 : 6,88 

<=> x = 819 / 430

b) 312 - x = 24,5 x 12,6 

312- x = 308,7

x = 308,7 - 312 

x = - 3,3

40:5/2=40*5/2=(40/2)*5=20*5=100

(x+1)+(x+2)+(x+3)+...+(x+100)=5750 
x + 1 + x + 2 + x + 3 + .... + x + 100 = 5750 
(x + x + x + ... + x) + (1 + 2 + 3 + ... + 100) = 5750 
100x + 5050 = 5750 
100x = 5750 - 5050 = 700 
x = 700 : 100 = 7