The literal translation is+ = 13. When translate in this way, you must check if it makes sense: you should check that such integer number n does really exist. For it, solve equation + = 13. Multiply both sides by 4*5 = 20 12n + 5(n+1) = 13*20 12n + 5n = 260 - 5 17n = 255 n = 255/17 = 15. You get this integer number n= 15, and it confirms that the translation does make sense.