35.149 = 30+5+0.1+ ____ +0.009
= 30 + 5 + 0.1 + x + 0.009
= 35.109 + x
x is the unknown value, so what number add to the 35.109 gives 35.149? That's what we're after.
35.149 = 35.109 + x
- 35.109 both side
0.04 = 0 + x
Thus, x = 0.04
(35.109 - 35.109 cancles each other & we get x on its own because 0.04 - 0 gives you 0.04)
Hope this helps!