Question 155967
You start with an equilateral triangle with sides of an unknown length. So make that length x and draw that triangle. Now draw another triangle with lengths x+20,  x+30, and x+40. Now calculate the perimeter of each. The perimeter of the original triangle is 3x. The perimeter of the larger triangle is x+20+x+30+x+40, or 3x+90. Now look at what else you know. The perimeter of the larger triangle, 3x+90, is between two and three times the original, or between 2(3x) and 3(3x), or between 6x and 9x. So you have an inequality: 6x < 3x + 90 < 9x. So you really have two inequalities here, 3x+90>6x and 3x+90<9x. Solve each of these for x, and you will find that x is between two numbers. (Now you need to find those numbers!)