Fifteen more than a number is ten. What is the number?

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Answer 1 · 2016-09-14T17:10:24

#x=-5#

Let the number be #x#

Write an equation to express the information.

#x+15 = 10" "larr# solve the equation

#x+15-15 = 10-15#

#x=-5#

Answer 2 · 2016-09-18T15:11:41

#-5#

Breaking the question down into its component parts:

Let the unknown number be #square larr" a sort of algebra"#

Fifteen more than:#->square +15#

is 10:#" "color(brown)(->squarecolor(green)(+15)=10)#

'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
But #color(green)(15)# can be written as #color(green)(5+10)# so write as:

#color(brown)(square color(green)(+5+10)=10)#

So we need to 'get rid' of the 5 on the left.

Suppose we change #square# to -5 we then have

#-5+5+10=10#

#0+10=10#

So the unknown number #square = -5#