Hey there, first off, the answer is +250. When you can't just look at a table and decipher the relationship between input and output you need to put in the grunt work.
Does the number increase when put into the machine? Yes! So it doesn't make sense to subtract. Any answer where you are subtracting from the original number can't be correct.
Okay, so we know the number increases when it goes into the machine, but what actually happens.
1 --> 251
2 --> 252
3 --> 253
4 --> 254
We can notice a pattern that all of the inputs are single digits, and the outputs are the single digit added to the quantity 250 - thus giving us our answer.
