A job has 90 steps i want to execute only step7 and step15....?

In the jobcard, Give restart=step7, code cond=(0,le) from
step8 - step14, Give a null stmt(//) after step15. That's
it your requirement is met.

Simplest way:
There is an utility IEBEDIT which is used to execute only
few particular steps from 'n' steps in a jcl.

//JOBCARD
//STEPNAME EXEC PGM=IEBEDIT
//SYSUT1 DD DSN=MYPRGM.DSN,DISP=SHR
//SYSUT2 DD SYSOUT=(*,INTRDR)
//SYSIN DD *
EDIT TYPE=INCLUDE STEPNAME=(STEP7,STEP15)
/*
//

MYPRGM.DSN = The name of the dataset that has the actual
JCL.