the bag is 200g
total weight with oranges is 1400g
deduct the bags weight from total weight
1400 - 200
1200g
this is the weight of the three oranges
so each orange would be
1200 ÷ 3
400g
Pumping iron: 10x+90=0
Fit for life: 15x+50=0
Set them equal to each other. 10x+90=15x+50. Solve for x. Subtract 50 from both sides, get 10x+40=15x, then subtract 10x from both sides, getting 40=5x. Divide 5 from both sides to isolate x, x=8. After 8 months the two gyms will cost the same.
The derivative is -6sin(3x)