Question

when marginal utility deacreases tu increases at diminishing rate why?

Answer 1

As the number of oranges consumed increases marginal utility decreases because total utility increases at a diminishing rate. In fact, marginal utility is the change in total utility associated with a small change in the consumption of a commodity and is measured by the slope of total utility curve

Answer 2

MU means additional utility on account of additional unit

When MU decreases...it only means less and less of MU is being added in TU... that's why TU increases but at decreasing rate