怎樣清楚計算下面問題既step?

The production cost of a desk includes the cost of materials of $500 and the labour cost of $300. If the cost of materials decreases by 20% and the labour cost increases by 10%, find the percentage change in the production cost of the desk.

The total production before
500 + 300 = $800

The total production now
$500 x (1-20%) + $300 x (1+10%)
= $500 x 0.80 + $300 x 1.1
= $400 + $330
= $730

The percentage change
730/800 x 100%
= 91.25%

It has decreases (100%-91.25%) = 8.75% in production cost of desk

10%+20%=30%


500+300=cost of the desk x 30%
800over30%=cost of the desk
cost of the desk=1142.85($)