Answer:
 the way to work these types of problems is to create a table. The table will have three rows and three columns.
 
The table:
 
Let x represent the kilograms of nuts, then 138 - x will represent the kilograms of bolts.
 
Product     Amount        Cost            Total
--------------------------------------------------------
   Nuts          x               .60         .60 * x
 
   Bolts      138 - x          .66         .66 * (138 - x)
 
   Mix          138             .65         .65 * (138)
 
 
Step-by-step explanation:
The equation:
 
The nuts plus the bolts must equal the mix or:
 
.60 * x + .66 * (138 - x) = .65 * 138
 
I do not like to work with decimals, so multiply each term by 100:
 
60 * x + 66 * (138 - x) = 65 * 138
 
Remove the parenthesis and combine like terms:
 
-6x = -66 * 138 + 65 * 138
 
-6x = -138
 
Divide both sides by -6:
 
x = 23
 
Solution:
 
The mixture contains 23 kilograms of nuts.
 
The mixture contains 138 - 23, or 115 kilograms of bolts.