the equation to your question would be P =10(c)+20. meaning 10 pencils in each box multiplied by the number of boxes (c) plus the 20 pencils the teacher gives her will equal the number of pencils (P) she has altogether. (C stands for the number of boxes and P stands for pencils)
so lets say she has 3 boxes. well, c stands for boxes soooo...... all you have to do is plug in 3 were c is. giving you this equation P = 10(3)+20. then all you have to do is solve. P = 10(3)+20
10 times 3 = 30 so P = 30+20
then add 30 and 20 P = 50 she has 50 pencils altogether
(remember this is just an example, 50 pencils is NOT the answer to your question. but you can use the equation i gave you to find your own answer!) :) Hope this helps!!
