This is about as hard as it gets when adding in your head. Anything harder it would be expected that written methods would be used. When adding two amounts such as £2.47 + £3.53 in your head most people begin with the pounds – the opposite of written methods. I would look at this sum and add the pounds and ten pences making £5.90 then add on the units, 7 first making £5.97 then the 3p making £6.00
Two other exercises here: adding £4.99 by adding £5 and subtracting one penny and adding £9.99 by adding £10 and subtracting one penny.
Free Y6 maths worksheet: Addition of decimals (pg 1)
Free Y6 maths worksheets: Addition of decimals mentally (pg 2)