If you were asked to design the most efficient nuclear fission reactor possible, what ratio of u-235 to u-238 would you use? explain why?

Please try this one

70:30 ratio respectively. 70% U-235 and 30% U-238 because I wouldn’t be able to control the reactor if the whole reaction happened at once. If I was making a bomb, I would make it to be that there are no U-238 nuclei and use all U-235. The U-238 would get int he way of the reaction.