tFrame = 30
cItems = 40
interval = tFrame / cItems = 0.75

NP_MATH = interval * (currentId-1) + interval

startFrom = 1

currentId = 1 ->  = NP_MATH = 0.75
currentId = 2 ->  = NP_MATH = 1.50


startFrom = 5

currentId = 5 -> = tFrame
currentId = 6 -> = MATH_IZ_HARD = 0.75
currentId = 7 -> = MATH_IZ_HARD = 1.50