This study aims to examine the differences in numerosity estimation on a right-to-left number line between second- to fourth-grade students and undergraduate students, together with whether number-line estimation is related to basic arithmetic tasks (addition and subtraction). Hence, 53 Arabic-speaking children and 63 Arabic-speaking adults performed a paper-based number-line estimation task and also an arithmetic task. Number-line estimation was represented either by the percentage of absolute errors in positing the quantities on the line, by the accuracy, or by the "r" score for linearity of representation. The results show that, although children's addition (but not subtraction) performance resembles that of adults, the correlations vary between number line and arithmetic scores. Addition scores are positively correlated with error percentages in the children's number-line tasks, while subtraction is negatively correlated with adults' error percentages. These differences are assumed to be attributed to various uses of recall and procedural strategies.