Here, We have to do the addition. We know, addition is quite simple but our main motive is "My Mental" (without any use of pencil & paper)
So, Your inputs are: 1.25 + 0.35 + 3.75
Now, we know, 0.25 + 0.75 = 1.00 so we will add 1.25 & 3.75 first which would be equal to 5.00 and 0.35 would make 5.35
In short, Your Answer would be: $5.35
Hope this helps!