Let us assume that 5k+5 is a multiple of 4.
⇒5k+5=4t
⇒5k=4t−5 - - - (1)
Let's check if the same holds true for k + 1.
5k+1+5=5k.5+5
= (4t - 5) 5 + 5
= 20t - 25 + 5
= 20t - 20 = 4(5t - 5)
This means that 5k+1+5 is a multiple of 4.
Hence, our assumption was correct.