Are you carrying too much on your back at school? You're not alone. Back experts(专家) in the USA are worried about that young students are having back and neck problems because they are carrying too much in their backpacks .
    “It hurts my back when I run,” said Ebelin Reyes, a student in Virginia. “It's hard to get up the stairs with my backpack because it's too heavy.” Students have to carry heavy backpacks on their backs for a whole week’s study. Ebelin is one of them. They have   regular(固定的)backpacks with two straps(带子) to carry them, but a number of students with heavy loads(负担) have switched to rolling backpacks. The backpacks have wheels and can roll on the ground. Shirley Park's backpack weighs 10 kilos, and she said, “I'll change to a rolling backpack because I am starting to have back pain.”
     How much is too much? Experts say that students should carry no more than 10% to 15% of their own body weight. A few students have had a good idea to lighten(减轻) the load: less homework.
Doctor's suggestions:
Lighten the load. Take home only the books the students need that day.
(2) Wide straps are better. They can send out weight over your shoulders (肩)and be sure to wear backpacks with two straps.
(3) The heaviest things should be packed closest to the back.
(4) Bend(弯曲) both knees when you pick up the pack, don't just bend over the waist(腰).

已知：一组自然数1，2，3…k，去掉其中一个数后剩下的数的平均数为16，则去掉的数是____ ．