Background: Base-ten thinking (BTT) -- children's ability to reason in tens and ones is a crucial measure of Foundation Phase learners' mathematical performance in South Africa. Aim: The study looks at the six learners using BTT to solve additive tasks through two different assessments. Setting: Six purposely selected Grade 3 learners in Johannesburg township school. Methods: The study used two different assessments to look at six learners' mental strategies for solving additive tasks. The first assessment analysed how additive tasks were solved by learners before and after an intervention (Mental Starter Assessment Project [MSAP]). The second assessment instrument (Learning Framework in Number [LFIN]) focussed on how learners (two high achievers, two average achievers and two low achievers) solved particular counting activities when solving additive problems. Results: The findings demonstrate that learners who can count efficiently, partition ones and tens, work with groups of ten and understand number relationships when solving addition problems, operate with high levels of BTT. Conclusion: The study showed that well-designed test items are crucial for assessing and enhancing learners' understanding of BTT. Contributions: This research offers insights into assessment practices that assist teachers in identifying BTT in resource-constrained settings.