An investor has 98 000 to invest. If part is invested at 5% at the rest at 10% how much should be invested at each rate to yield 8% on the total amount? How much will be the interest on the total?

If x is at 5%, the rest (98000-x) is at 10%. So, just add up the interest:

.05x + .10(98000-x) = .08(98000)