Answer
Find out the how many paving stones will he need to install to finish.
To proof
As given
The pave the way company is installing a new walkway
installer uses 13paving stones for each foot of walk way
he needs 117 paving stones to complete the progect and has 78 paving stones so far
let us assume that the number of paving stones will he need to install to finish be x.
Than the equation
x + 78 = 117
x = 117 -78
x = 39
39 paving stones will he need to install to finish
Hence proved