Let the weights of the cherries held by Kristin and Doris be k and d, respectively. Kristin holds 4.2 kg fewer cherries than Doris: k = d - 4.2.
After Kristin sells 0.85 kg of cherries, k - 0.85 represents her holdings. Doris' holdings are twice Kristin's, or d = 2(k-0.85).
Find Doris' holdings. Eliminate the variable k:
Substitute d - 4.2 for k in d = 2(k-0.85): d = 2( [d - 4.2] - 0.85).
Perform the indicated multiplication now: d = 2d - 8.4 - 1.7.
Grouping like terms: 10.1 = d. Doris holds 10.1 kg of cherries.
