42 ÷ 396 You could use 40 ÷ 400, which would give you 0.1 as an estimate. You could also use 42 ÷ 420, which would also give you 0.1 as an estimate.
By the way, by a pair of "compatible numbers", it means when you can easily work out the sum by calculating the first digits multiplied or divided together, then add the zeros.
For example, with 40 ÷ 400, we know that 40 ÷ 40 is 1, so we just put the other 0 infront of the decimal point to make it 0.1.
If it was multiplying (2 x 600), you could work out 2 x 6 (12), then add the 0's to get 1,200
