This is a Countdown Practice game.
Also check out the Countdown Solver. It's good.
Use the "Vowel" and "Consonant" buttons to pick your letters, or "Conundrum" to practice a conundrum, then you have up to 30 seconds to find your words (use the working area if you like), then you can use the "Check word" box to check your best words, and "Show answers" to see the top answers.
Decide how many large numbers you want, or use "Random" mode, then see if you can solve the generated game before your 30s are up.