Difference in size 4th grade math.
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1.25 or 1 1/4 because:

you take the size of the biggest button - it from the smallest button.

so:
 1 1/2 - 1/4 = 
(Its so much easier in decimal form so lets change it into decimal form)
  (I know that 1/2 = .5 so what does 1/4 = ? Well .5 = .50 so what is half of 50? 25 is. so 1/4 = .25 so now you take those two numbers and mince) 
0.50 - 0.25 = 0.25 
Now put the 1 on it:
1.25 
Know change it to a fraction:
1.25 = 1 1/4
       
                                           

                                             HOPED THIS 
                                               HELPED

Line ab is y = -7/5x + 56/5i don't understand  where point d comes into this.