.
D = 3T (1) ("David is thrice as heavy as timmy")
P = T + 3.6 (2) ("patrick is 3.6 kg heavier than timmy")
D = P + 25.8 (3) (" david is 25.8 kg heavier than patrick")
Now substitute D = 3T into equation (3), based on (1).
Also, substitute P = T + 3.6 into equation (3), based on (2).
You will get then
3T = (T+3.6) + 25.8.
Simplify and solve for T
3T = T + (3.6 + 25.8)
2T = 29.4
T = 29.4/2 = 14.7.
Then from equation (1), D = 3*14.7 = 44.1.
ANSWER. David's mass is 44.1 kilograms.
Solved.
It can be solved by other ways, too, but this one is the simplest.
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It is just from sixties of the past century (i.e. more than 50 years) kilograms are not used to measure weight.
Kilograms are used to measure mass,
but for weight the International System of measuring units (SI) exluded kilograms as the measure of weight / force, replacing them by Newtons.