book Maths Puzzle Numerical Ability

Can you help me with this problem:

1 1/15 + 3 3/10 - 2 4/5

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book Other Question

is there a trick to finding a prime number Vern invested $11,000. Part was invested at 15% and part at 13%. If the total interest at the end of the year is $1,610, how much did he invest at 15%?