5th Grade Math Factor Puzzle?

I have a square box with 3 numbers inside, I need to find the third number. 2 10
? 15
There are no other instructions. My daughter is in 5th grade math. Does anyone have any suggestions, it doesn’t refer to multiplication, addition, or subtraction?

it is goes 2, 10, 15
the pattern would be you multiply by 5, add 5.

but if theres one between the 10 and 15 i have no idea sorry

Grade 1.Reading. Word puzzles