The answer is B
Explanation:
As shown in the given figure, ΔDEF is a right-angles triangle, where ∠DFE = 90°, DF = 15 units and EF = 20 units.
We are to find the length of DE.
ΔDEF is right-angled at ∠DFE.
So, using Pythagoras theorem, we have
\begin{lgathered}DE^2=DF^2+EF^2\\\\\Rightarrow DE^2=15^2+20^2\\\\\Rightarrow DE^2=225+400\\\\\Rightarrow DE^2=625\\\\\Rightarrow DE^2=25^2\\\\\Rightarrow DE=25.\end{lgathered}
DE
2
=DF
2
+EF
2
⇒DE
2
=15
2
+20
2
⇒DE
2
=225+400
⇒DE
2
=625
⇒DE
2
=25
2
⇒DE=25.