N+1=n×5-15

N + 1 = N * 5 - 15

is the same as

N + 1 = 5N - 15

Add 15 to both sides.

N + 1 + 15 = 5N - 15 + 15

The -15/+15 on the right cancel out. 1 + 15 simplifies to 16.

N + 16 = 5N

Subtract N from both sides.

16 = 5N - N

5N - N simplifies to 4N.

16 = 4N

Divide both sides by 4.

16/4 = 4N/4

The 4s on the right cancel out. 16/4 = 4.

4 = N

So, N = 4.

That is an excellent answer anon