Step-by-step explanation:
% calculations are totally easy, if you remember to always find and the use 1%.
100% = $2.85
1% = 100%/100 = 2.85/100 = 0.0285
the price difference was
2.91 - 2.85 = $0.06
how many % are these $0.06 compared to yesterday's price ?
that is how many times 1% can fit into this number.
0.06 / 0.0285 = 2.105263158...%
so, rounded this is 2.1%
