Hold ALT and click on the stopwatch of your B layer's time remap. Use the pickwhip to select your A layer's time remap property, then add the following code to the end of the expression:
alayer.time%framesToTime(15)
The bit before the % should be generated for you by the pickwhip.
Personally, I'm never 100% confortable with modulus, I like it more spelled out, so linear would also get you there:
linear(alayer's time,0,100,0,15)
That takes the A layer's time in seconds, and if it is 0 seconds, set B time's to 0, and if it's 100 seconds, set B to 15 (and map the range of values inbetween accordingly). It's more long-winded and you have to know the final time for A, but I think it spells it out more when you're learning.
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