How should I downsample my sensor?
If you can assure that the physical signal being measured by A has no detectable components above 50Hz, then you could take 1 every 10 samples.

However, if you have CPU power available, it is much better to filter the signal (lowpass, with stop-band starting below 50Hz) before downsampling. This way, components above 50Hz won't be aliased down to your working bandwidth. Also, this process will reduce quantization noise from the ADC, which is a good thing.