[masraum]

Originally Posted by JD159

How do you know where to put the signs?

mental trial and error

the leading interger 2 must be factored (2 and 1, 2*1 = 2). The -15 must be factored, (options are -1,15, 1,-15, -3,5 or 3,-5)

You then have to multiply and then add to get the middle interger (+7)

so

(2*-3) + (1*5) = -1

(2*3) + (1*-5) = 1

(2*-5) + (1*3) = -7 (close, but not quite)

(2*5) + (1*-3) = 10 + -3 = 7 --> (+7x)