Mission Measurement!

Raj is a rice-seller, who has $4$ weights and a scale-balance. The values of the 4 weights are $a$, $b$, $c$ and $d$ such that all the values are positive integers and all are different numbers. Also, $a+b+c+d=40$

Also, using a few or all of the weights, he can measure any integer weight of rice from $1$ to $40$ i.e he could either use $1$ weight or a few or all the weights as much it requires to measure a certain weight using the scale-balance. If $a then what is $a+d$.

Note: In case you don't know about how weights are measured in a scale balance, here is an example. Suppose you have $3$ weights like $9$ kg, $10$ kg and $7$ kg, so you could either measure some weights by using all or few, like using all you can measure $26$ kg by keeping all the weights on one side and the thing to be measured as $26$ kg on the other side or using a few measure $2$ kg by keeping the thing to be measured along with the $7$kg on $1$ side and the $9$ kg weight on the other side.

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