A triangle is to be changed by increasing the length of its base by 40% and decreasing the length of its height by 40%. Which of the following must be true about the area of the triangle?

Question

anonymous · Answer

if you get confused by a problem like this, one nice gimmick is to try it with actual numbers
for example, lets say the base is 100 and the height is 100 so the area is $$5000$$

anonymous · Answer

increase  the the length by 40% make it $140$ decrease the height by 40% makes it $60$

anonymous · Answer

then the new area is 
$$\frac{140\times 60}{2}=4200$$