You'll calculate the amount it was decreased by :
15% is also 15/100
15/100 * 42000
The two 0's in 100 cut 2 of the 0's in 42000 making it :
15/1 * 420
15* 420 = 6300
So, 6300 is the amount that the price is decreased by, but the question says the price of the PC (personal computer) after the decrease has happened not the price it decreased by :
So,
42000 - 6300 = 35700.