Thats not too tough - basic performance calcs. Doubt the figures you will get at selection will be quite so oblique - generally they tend to be more rounded numbers. Anyways, for that example I would do the following.
42 is close to 50, and you know 350 is divisible exactly by 50:
350/50 = 7
I would then multiply this by the number of miles each 42 litres will take you:
7 x 120 = 840 miles
With this, check against the choices you are given. If there is a clear cut answer, then there you have it. If there are a couple close by, you need to go a little further....
Via long division:
350/42 = 8.3
Then through long multiplication:
8.3 x 120 = 996 miles
The crude estimation works out to be nearly 20% different so I guess it's all down to the factors you choose. Take each question as it comes and decide how precise you need to be - you dont necessarily have enough time to labour over precise calculations for all the questions (much like on the flightdeck).