In.ignore question

I have an input file:
3
Name1     X9/X9/90X-9XF99/9
Name2     XXXXXXXXXXXX
Name3     7/9/X80XX71--XFF

But how would I get it to ignore everything up until (not including) the 7 in line 4? I've tried a lot of different things but I guess I'm not too familiar with how "ignore" works. I thought you did In.ignore("number of characters", "a certain character"). Thanks for the help.
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