Could someone help me out with this please?

All per-unit concepts rely on ratios,meaning,to provide a type of measurement

Isaac wants the equation below to have no solution when the missing number is placed in the box. which number should he place in the box?

Maria has $10,000 to invest in two accounts.he decides to invest one part in an account with 5% interest and the rest in an account with 8% interest. at the end of the year he has earned $702,50 in interest.¿how much did maria invest in the 8% account?