Logic question problem

Question

$$49\blacktriangle\to13$$

$$56\blacktriangle\to11$$

$$47\blacktriangle\blacktriangle\to\,\,?$$

Choices: A) 82, B) 54, C) 28, D) 16, E) 2

I tried some combinations but i just cannot understand it.What does a triangle stand for.

Answer 1

It looks like $\blacktriangle$ is a function that takes a number and adds up the digits of that number (4+9=13, 5+6=11). So $47 \blacktriangle \blacktriangle$ would apply that to 47 and then again to the result. I'll leave it to you to figure out the rest.