3.9/x=100/78
(3.9/x)*x=(100/78)*x - we multiply both sides of the equation by x
3.9=1.28205128205*x - we divide both sides of the equation by (1.28205128205) to get x
3.9/1.28205128205=x
3.042=x
x=3.042
now we have:
78% of 3.9=3.042
