First, let's write #1 1/8" cups"# as an improper fraction:

#1 1/8 = 1 + 1/8 = (8/8 xx 1) + 1/8 = 8/8 + 1/8 = (8 + 1)/8 = 9/8#

Let's call the amount of fiber we are looking for: #f#

We can then write the relationship and solve for #f#:

#f/(9/8"cups") = (3"g")/(1/2"cups")#

#color(red)(9/8)color(red)("cups") xx f/(9/8"cups") = color(red)(9/8)color(red)("cups") xx(3"g")/(1/2"cups")#

#cancel(color(red)(9/8)color(red)("cups")) xx f/color(red)(cancel(color(black)(9/8"cups"))) = color(red)(9/8)cancel(color(red)("cups")) xx(3"g")/(1/2color(red)(cancel(color(black)("cups"))))#

#f = color(red)(9/8) xx(3"g")/(1/2)#

#f = ((27"g")/8)/(1/2)#

#f = ((27"g") xx 2)/(8 xx 1)#

#f = (54"g")/8#

#f = 6.75"g"#

There are close to 7 grams of fiber for the oatmeal left in the box.