here's an example of %
20% = 0.2
2% = 0.02
when multiplying with decimals in place you always add up the two periods, 155.8 × 0.2 = 31.16
since the dot is one over for both you add 1 + 1 which equals 2, then when you get 31.16 you move over 2 decimal points, making it 31.16
btw the answer is when you subtract 31.16 from 155.80 which would be 124.64$ for your final cost.