834/3 isn't something everyone can do in their head
BUTIt's easy to see it has to end in 8. As 8x3 = 24, and no other single digit multiplies by 3 to end in 4.
So, two third will end in 8x2 = 16... a SIX!
So, we're taking 6 away from some number or other... which number? Some large odd number times something that ends in 5. So that number will ALWAYS end in five.
We're subtracting something ending in 6 from something ending in five.
Ergo, without any maths harder than 8x3, we know our answer ends in 9. There's only one answer that ends in 9, so we win!