If the percentage of calories from fat has decline since 1971 why are so many american overweight

I have not researched into this area but I can think of a few explanations.
1) Food prices have decreased due to globalisation which has lead to an increase in portion sizes and overconsumption. Food prices are much cheaper these days as globalisation has allowed us to grow and produce food much cheaper in LICs (low income countries). This means Americans are more likely to consume more food as it is cheaper.
2) Fat is not the only cause of obesity: In the 1970s-1980s the popularity of low fat "health" foods skyrocketed (and has maintained popularity to this day). To compensate for the lower fat the amount of sugar was increased. Sugar is as bad or even worse than fat when it comes to causing obesity so replacing fat with it does not stop the rise of obesity.

This is a very interesting question and I could write many many more hypothetical answers so this is just scratching the surface.

Hope it helps.

Schools are being forced to embrace computers. unlike their "digital immigrant" teachers, these students are "digital natives", born into a world where technology is an integral part of everyday life. almost every student has access to a multitude of computers, laptops and tablet devices. it would be crazy to assume an assignment would  notbe written on computer in this day and age.many schools now have a 1-to-1 computer policy, and many districts and states in the united states have mandated computers or ipads across the state for every student,  including los angeles, the second largest school district in the country.but the thing is, when they tried it in la, it wasn't the success they hoped for. that's because the tablets weren't used as much as they expected. rather, it appeared there was a gap between the digital native's love of technology and their use of it in the classroom.this conflicted relationship with computers is also underscored by much of the literature on education. it is also highlighted in professional development and conferences such as  edutech.