Create a proportion. 3 1/2 is what percent of 4 as x is what percent of 100%. Then subtract the x amount from 100% to find % to cut back.
x= percent 3.5 is of 4
3.5/4 = x/100
cross multiply
(3.5*100)= (4*x)
350= 4x
87.5= x
Percentage to cut back:
=100% - 87.5%
=12.5% amount to cut back
ANSWER: The recipe needs to be cut back by 12.5%.
Hope this helps! :)