A material is purchased for Rs. 600. If one fourth of the material is sold at a loss of 20% and the remaining at a gain of 10%, Find out the overall gain or loss percentage

We need to get the Total selling price to solve this question. Because after getting selling price we can get profit or loss, then we can calculate profit% or loss%
So lets solve this:
Price Received by selling one fourth of the material at a loss of 20% =
$(1/4) * 600 * (80/100) = Rs. 120$
Price Received by remaining material at a gain of 10% = 
$(3/4) * 600 * (110/100) = Rs. 495 [Note: 1-(1/4) = 3/4]$
Total Selling Price $= 120 + 465 = Rs. 615$
Profit $= 615 - 600 = 15$
                         $$\begin{gathered} Profit\%=\left(\frac{Gain}{Cost}*100\right) \% \\ =\left(\frac{15}{600}*100\right) \% \\ =\frac{5}{2}\% =2\frac{1}{2}\% \end{gathered}$$