These problems feature two-digit problems and sums. The first number can be anything between 10 and 98; the second one can be anything from 1 up to the largest number that won't result in the sum being higher than 99. Some problems will have single-digits being added to double-digits; this is intentional.

Although you likely can do it, mental mathematics get harder here. One trick applicable from here on is to convert the problem into a simpler one. Look out for pairs of numbers that are known to add up to 10 (two 5s, a 4 and 6, a 3 and 7, and so on). Another way you can do this is to solve intermediate problems - for instance, you can add just enough to one side (while subtracting it from the other) to make it a round number (that is, one ending in 0). Then the two modified numbers should be easier to resolve. It's also helpful to consider whether the addition causes any carryovers. If there are none, then the sum can be treated as the concatenation of a series of 1-digit problems.