The area under a complete density curve is equal to 1. So, the percent of the total area under a portion of the curve is equal to the area under that part of the curve.

We are looking for the area under the density curve when x < 1.

We can use the area formula for a triangle to find this area:

Area = \frac{1}{2} \times base \times height = \frac{1}{2} \times 1 \times 0.1 = 0.05

So the percent of the area under the density curve where x \lt 1 is 5%.

I found an answer from www.khanacademy.org

Density Curves (video) | Khan Academy

An introduction to density curves for visualizing distributions. ... Imagine that you have a continuous data set (like the quantity of water that ... I understand that the area under the curve is the probability of getting an X in that interval. ... AP® is a registered trademark of the College Board, which has not reviewed this resource.

For more information, see Density Curves (video) | Khan Academy