Question taken from RD sharma
Class 10th
Chapter no. 4
Chapter name:- Triangles
Exercise :- 4.3
This is very basic and important questions.

In this question we have  a ∆ABC,

AD is the bisector of ∠A, meeting side BC at D.

Also it is given that BD = 2.5 cm, AB = 5 cm and AC = 4.2 cm,

Now we have to find DC

Understanding and learning CBSE maths
RD sharma, DHANPAT RAI publication