Step-by-step explanation: once he gains 40 yards he is at the 10 yard line then once he goes back 5 he is at the 15 yard line which is in between the 20 and ten
The 40 yard gain would bring him to the 10 yard line (he caught it at the 50 yard line, so 50 - 40 = 10), but the 5 yards lost due to the tackle and push back would bring him to the 15 yard line (10 + 5 = 15).